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The Oil Palm, Fourth Edition

R.H.V. Corley, P.B. Tinker

Copyright © 2003 by Blackwell Science Ltd

Chapter 12

Diseases and Pests of the Oil Palm

Until World War II it appears that the oil palm was largely free from serious diseases and pests (Hartley, 1988), but as the area under the crop has expanded, there have been serious, and at times devastating, out- breaks of disease in several parts of the world. Of greatest importance have been the devastation caused by Fusar- ium wilt in several parts of Africa, the considerable losses sustained through dry basal rot (Ceratocystis) in Nigeria and associated with Ganoderma in old and replanted areas in Asia, and the attacks of fatal yellow- ing and sudden wither on new plantations in Latin America.

Diagnosis and prevention or cure of some of these dis- eases have proved difficult. It is not clear in some instances whether a pathogen is involved, or whether the symptoms are a disorder caused by some abiotic factor. The fact that some, perhaps most, diseases only become serious under certain predisposing environmental con- ditions may further complicate matters. One approach has been to search for resistance to, or tolerance of, the diseases, both within Elaeis guineensis material and in interspecific hybrids with Elaeis oleifera. Good progress has been made in relation to Fusarium wilt with E. guineensis, and there is scope for this approach with Ganoderma. Hybrids may be tolerant to fatal yel- lowing, but poor yields limit their value at present.

Attacks by one pest, the leaf miner Coelaenomenodera lameensis, have been serious in West Africa, while sporadic defoliation has also been caused by caterpillars and bagworms of various species in Malaysia and Latin America. With the steadily increasing areas under oil palms there has been a general increase, particularly in Asia and America, in the number of pest species recorded; several natural orders are represented, but particularly the Lepidoptera and Coleoptera.

A comprehensive work on diseases was provided by Turner (1981). For pests, Wood (1968a) gave much information of general application on ecology and con- trol, while detailed information on African and American pests is to be found in special issues of the journal Oléagineux (Genty et al., 1978; Mariau et al., 1981), and also in Mexzón and Chinchilla (1993). Readers should

refer to specialist works if pesticide treatment is recom- mended. We have not given details here, because new pesticides are constantly being developed and older ones withdrawn.

Descriptions in this chapter must necessarily be con- densed, but reference is made to original papers where greater detail can be found. Nutritional disorders are described in Chapter 11 and this chapter will therefore deal with conditions caused by pathogenic organisms, with important disorders of unknown cause, and with insect and other pests causing damage to the palm.

12.1 DISEASES AND DISORDERS

It is convenient to deal with diseases according to the stage of growth at which the palm is attacked and the organs affected. However, there is much overlap and some diseases have been rather arbitrarily listed for a particular stage of growth. For example, Cercospora is primarily a nursery disease, but also occurs in young palms in the field. Ganoderma may be characteristic of one age in some regions and of a different age in other regions, while other diseases may have different mani- festations at different ages. Information on a number of ‘minor’ diseases is summarised in Table 12.1. Some of these caused serious losses at one particular time and place, but have not been a problem since. Several were observed in the early days of oil palm cultivation, when fertiliser inputs were low or absent, and poor nutrition was probably an important predisposing factor.

Pathologists set much store by Koch’s postulates, conditions which must be met before the cause of a dis- ease is considered proven. In particular, the require- ments that typical disease symptoms be obtained after inoculation with a supposed pathogen, and that the pathogen then be reisolated from the inoculated plants, have not been met for several oil palm diseases. Even where Koch’s postulates are proven, predisposing envir- onmental factors may still be important.

391

Table 12.1 Minor oil palm diseases: diseases either causing little economic damage, or of rare occurrence

Disease

Symptoms

Cause

Location

Ref.

Comments

 

 

 

 

 

 

Nursery diseases

 

 

 

 

 

Anthracnose

Necrotic lesions, slow growth

Various (see Ref. 1)

Widespread

1, 2

 

Seedling blight

Elongated spots with yellow halo

Curvularia eragrostidis (?)

Malaysia

1

Treatment similar to

 

 

 

 

 

Cercospora

Cylindrocladium

Brown lesions with white centres

Cylindrocladium

Ivory Coast

1

 

 

 

macrosporum

 

 

 

Leaf spotting

 

Various (see Ref. 1)

Widespread

1

See Ref. 1 for control

 

 

 

 

 

recommendations

Nursery spear rot

Rotting of median leaflets of

Phytophthora sp.

Congo

3

Not serious enough to

 

the spear leaf

 

 

 

need control

Corticium leaf rot

Rot at base of unopened leaves

Corticium solani

Congo, Malaysia

3

See Ref. 1 for control

 

 

 

 

 

recommendations

Dry bud rot

Yellow patches on leaves,

Unknown

Ivory Coast

4

Similar disease transmitted by

 

then dry rot of spear

 

 

 

Sogatella in coconut (see also

 

 

 

 

 

Section 12.1.5.5)

Chlorotic ring

Conspicuous mottling

Potyvirus, related to

S. America

5, 6

 

 

 

sugarcane mosaic virus

 

 

 

Mature palm leaf diseases

 

 

 

 

 

Necrotic spot

Brown spots, orange halo,

Cercospora elaeidis

Africa

7, 9

Different strain of C. elaeidis

 

premature withering

 

 

 

from that causing freckle

 

 

 

 

 

(Section 12.1.2.1)

Crusty spot

Orange spots with black

Parodiella circumdata

Africa

7

Mainly affects oldest leaves

 

crust in centre

 

 

 

 

Genetic orange spotting

 

Viroid?

Widespread

8

Transmission not

 

 

 

 

 

demonstrated

Algal leaf spot

Pin-point yellow spots on

Cephaleuros virescens

Widespread

1, 9

Effect on yield unknown

 

upper surface

 

 

 

 

Other mature palm diseases

 

 

 

 

 

Armillaria trunk rot

Similar to vascular wilt,

Armillariella mellea

Congo

10, 11

Incidence has decreased

 

but leaf bases rot

 

 

 

since the 1950s

Basal decay

Sudden death of leaves,

Unknown

Africa, Malaysia

1

Rare

 

following trunk rot

 

 

 

 

Charcoal base rot

Black rot at base,

Ustulina sp.

Malaysia

1, 12

Pathogenicity not proven

 

leaves chlorotic

 

 

 

 

Stem wet rot

Internal rotting

Unknown

India

13

 

 

 

 

 

 

 

References: 1: Turner (1981); 2: Robertson (1956); 3: Kovachich (1957); 4: Renard and de Franqueville (1989a); 5: Rivera et al. (1996); 6: Morales et al. (2002); 7: Kovachich (1956b); 8: Hanold and Randles (1991); 9: Robertson et al. (1968); 10: Wardlaw (1950a); 11: Moureau (1952); 12: Thompson (1936); 13: Chander Rao (1997).

Diseases and Pests of the Oil Palm

393

12.1.1 Diseases of germinating seeds: brown germ

Symptoms and distribution: Brown spots appear on the emerging ‘button’. These spread and coalesce as the embryo develops, and the tissues become slimy and rotten. The disease may occur wherever seeds are being germinated.

Cause: Duff identified a variety of Aspergillus niger from diseased embryos in Nigeria, and demonstrated pathogenicity by inoculation and reisolation (A.G. Prendergast, pers. comm., 2001). This may not be the only cause, though: Turner (1981) listed 27 fungi asso- ciated with the disease, of which Aspergillus spp. and Penicillium spp. were most frequent. Many are second- ary invaders, as are bacterial species.

Control: Brown germ develops most readily under moist conditions at a temperature of 38–40°C; use of the wet heat treatment for germination (see Section 7.1) therefore encourages its spread. Although sanitary meas- ures in the germinator may reduce incidence, the best method of control is to adopt the dry heat treatment method of germination, since the seeds are dry when being heated at 39.5°C, and when germinating they are at around 27°C, a temperature that does not encourage the growth of the organisms.

12.1.2 Seedling leaf diseases

Turner (1981) notes the importance of nursery man- agement in minimising disease susceptibility. With adequate water supplies and balanced nutrition, nur- series in many areas remain largely free of serious dis- eases, and investigations into disease outbreaks may primarily involve examination of growing techniques, rather than a search for a pathogen. This emphasises the importance of predisposing factors in disease develop- ment, as mentioned above.

12.1.2.1 Cercospora leaf spot, or freckle

Distribution: Cercospora leaf spot is widespread throughout Africa but has not been reported in Asia or America. It is a disease of nursery seedlings which some- times starts in the prenursery and is frequently carried to field plantings, where it can survive for many years.

Symptoms: The youngest leaves of nursery seedlings become infected and minute translucent spots sur- rounded by yellowish-green haloes enlarge and become dark brown. Conidiophores emerging through the sto- mata in the centre of the spots, mainly on the under- surface of the leaf, produce conidia which give rise to

further, surrounding spots. This results in a freckled appearance, but later the lesions coalesce and the tissue dries out to become greyish-brown and brittle. The disease tends to become aggressive as the leaves age, and the process described above may proceed very rap- idly at certain periods of the year. In West Africa, this is usually the middle or end of the wet season, and in the following dry season the drying out of the older leaves is much hastened by Cercospora incidence.

Cause: Cercospora elaeidis. Proof of pathogenicity was obtained by Kovachich (1954) in Congo and Robertson in Nigeria (1956). For details of growth and reproduction of Cercospora in the host, the papers of these authors and of Weir (1968) should be consulted. Nitrogen manuring may cause a small increase in the incidence of freckle in the nursery, but potassium substantially reduces it. Small favourable effects of phosphorus have also been noted (Robertson, 1960).

Effect on yield: Duff (1970) showed that Cercospora could depress yields by more than 10% over the first 7 years of production. Even moderate attacks materi- ally reduced the green leaf area and might therefore affect early bunch production. Note, though, that quite severe defoliation during the first year in the field may have little effect on subsequent yield (see Section 12.2.1.2). Jacquemard (1998) describes the disease as depressing nursery growth, but not economically important.

Control: The obvious course is to try to eradicate the disease in the nursery and to prevent reinfection of the young seedlings in the field. Jacquemard (1998) recom- mended spraying with mancozeb and benomyl. In the nursery, if spraying is not done, all old dry leaves and any others badly infected should be removed by prun- ing. In the field, however, excessive pruning may reduce growth and delay flowering, whereas failure to prune may increase the severity and prolong the incidence of the disease. A compromise pruning standard suggested by Hartley (1988, p. 585) was to remove and burn any leaf that showed dead or badly necrotic areas over more than one-third of its total surface, but it is not clear whether this would control the disease.

Elaeis oleifera progenies planted in Africa have shown a marked susceptibility to the disease; interspecific hybrids are rather less susceptible. There are significant differences between E. guineensis progenies in Cercospora susceptibility (Robertson, 1963) and, since serious loss of crop through Cercospora attack in field plantings has been demonstrated, Duff (1970) suggested that breeding for tolerance would be worthwhile. With good control being obtainable with fungicides, however, it is doubtful whether breeding for tolerance can be justified.

394

The Oil Palm

12.1.2.2 Other seedling leaf diseases

Various diseases of minor importance or rare occur- rence are listed in Table 12.1. Some nursery diseases, for which many causes (including virus infection) have been suggested but none established, have been con- stant enough in their symptoms to acquire distinctive names. These include bronze streak, ring spot and infectious chlorosis (which, despite the name, appears

not to be infectious). Turner (1981) gives information on these.

Leaf distortions occur in young prenursery and nur- sery seedlings at the bifurcate leaf stage (Plate 12.1) and have been described as:

•leaf crinkle, in which the lamina between the veins is folded in lines across the leaf

Plate 12.1 Certain abnor- malities of nursery seedlings.

(A) Leaf Crinkle; (B) Leaf Roll and (C) Collante.

Diseases and Pests of the Oil Palm

395

•leaf roll, in which the lamina is rolled under the leaf, giving it a spiky appearance

•collante, in which the lamina between the veins becomes laterally compressed at a band about halfway along the leaf so as to form a constriction there (Gunn et al., 1961).

Other abnormal conditions that necessitate culling in the nursery are described in Section 7.2.2.4.

Malformed seedlings are not uncommon in the prenursery and are variously attributed to the after- effects of brown germ, or to incorrect orientation of the germinated seed at planting.

12.1.3 Seedling spear and bud rots

Several nursery spear and bud rots have been recorded, but none appears to be serious (Table 12.1).

12.1.4 Seedling root diseases: blast disease

Distribution: Blast has been a serious nursery disease throughout West Africa. It was particularly severe in the Ivory Coast and of considerable importance in Nigeria and Cameroon. The disease has also been recorded from Malaysia (Turner, 1966b), Indonesia, Brazil (Cardoso, 1961) and Colombia, and Turner (1981) considered that it could occur in any country where climatic condi- tions and nursery techniques are likely to favour its development.

Symptoms: The symptoms of blast disease were described in detail by Bull (1954) and Robertson (1959a). Affected seedlings lose their normal gloss and become dull and flaccid, the leaf colour changing successively to olive green, dull yellow, purple or umber (at the tips) and, finally, with full necrosis and drying out, to a brittle dark brown and grey (Plate XIA). Necrosis of the cen- tral spear is usual and death occurs in a few days. In a small percentage of cases the rot may not reach the growing point; the seedling then survives, but as a weak and unacceptable plant. In many of the roots of diseased plants the parenchymatous tissue within the hypoder- mis has been rapidly destroyed from the tip towards the stem base, the stele remaining loose within the hollow cylinder. When the rate of cortical rotting becomes greater than the rate of production of new absorbing roots, desiccation and death follow very rapidly.

Causes: Two quite distinct causes for this disease have been convincingly demonstrated. Robertson (1959b) showed that it was caused by a joint infection of the roots by two fungi, and satisfied Koch’s postulates in proof

of this. Subsequently, it has been demonstrated in the Ivory Coast that an insect vector is involved. Turner (1981) discussed the possibility of there being several causes of blast or of two apparently different causes being linked. The leaf symptoms are essentially those of acute water stress, and might be caused by any severe damage to the roots.

Robertson (1959b) isolated Rhizoctonia lamellifera from decaying cortical tissue in the roots, and a Pythium species, probably P. splendens, from primary infections of the root tips, where it was shown to penetrate the cells and cause their collapse. In laboratory experiments, Robertson (1959b) showed that the Pythium may be parasitised by R. lamellifera. In inoculation experi- ments, a mixed inoculum of Pythium and R. lamellifera produced more extensive root rotting, and the leaf symptoms were more pronounced than with either species individually. Inoculation with R. lamellifera alone was only successful when the roots had been artificially damaged, while with Pythium inoculation, damage was confined in the root tips. In all of these cases patho- genicity was established by reisolation of the organisms. It was concluded that R. lamellifera plays an important part in blast disease in the destruction of cortical tis- sues and that it gains access either through a prior inva- sion by Pythium sp., which it parasitises, or through root damage from some other cause. The Pythium species was thought to be important through its role as a pri- mary invader and its ability to penetrate the parenchyma cells and develop within them.

Subsequent to Robertson’s work, quite different results were obtained in the Ivory Coast, where the blast problem had always been severe. It was noted that plants grown in metal cages covered with mosquito netting showed less than 1% blast in comparison with 15% outside in unshaded areas. A polybag nursery trial compared a completely closed cage with very fine net- ting (to give the minimum shading effect), an open-top cage, plots treated twice weekly with parathion, and unshaded control plots which had natural grass between the bags (Renard et al., 1975). The results were as follows:

 

Treatment:

 

 

 

 

 

 

 

 

 

 

 

Completely

Open-top

Parathion

No

 

 

enclosed

cage

 

treatment

 

 

 

 

 

Blast (%) by

0

6

27

46

end of Dec.

 

 

 

 

 

Blast (%) by

2

9

35

63

end of Jan.

 

 

 

 

 

 

 

 

 

 

 

396

The Oil Palm

These results led to the hypothesis that an insect vec- tor was involved. Later, it was established that the insect was Recilia mica (Hemiptera: Delphacidae), for which the grasses Paspalum spp. and Pennisetum spp. were alternate host plants ( Julia, 1979). Julia found that R. mica moved to the palm nursery from surrounding grass only in October and November. De Franqueville et al. (1991) found that the insects were most frequent in November and December, and introduction of R. mica to caged palms gave the highest disease incidence at that time. The exact connection between R. mica and blast disease has not been determined. Renard (1981) showed that tetracycline gave good protection, suggesting the possibility that a mycoplasma is involved, but this has not been confirmed.

Control: The precautions that are taken in nurseries against blast are briefly discussed in Chapter 7. The effect of shade in reducing blast incidence has been estab- lished, but the provision of shade for large plants nearing the end of their nursery life has disadvantages, and in Nigeria generally proved unnecessary. A significant negative correlation was found between blast incidence and rainfall during the ‘short dry’ season in August and September (Robertson, 1959a). The blast season nor- mally extends from October to January, and experiments confirmed that irrigation during August and September substantially reduced blast incidence.

An interesting feature of blast disease is the import- ance of time of attack. It has been shown both in the Ivory Coast (Bachy, 1958) and in Nigeria (Robertson, 1959a) that a relationship exists between blast incidence and the age of the seedlings at the time of attack. If the seedlings are either very young (1–4 months) or old (11 months or over) at the beginning of the blast season, the casualties are few. De Franqueville et al. (1991) showed that seedlings with four or five leaves were more susceptible than older seedlings. In seedlings with two leaves the disease developed slowly, but eventually reached the same level as in plants with four or five leaves.

Desmier de Chenon (1979) found that the removal of grasses in the vicinity of the nursery reduced blast incidence; the application of aldicarb monthly from the start of the nursery was also effective and made it pos- sible to eliminate the shade which had always been found necessary in the Ivory Coast (Quencez, 1982).

The control measures for blast in West Africa may be summarised as follows.

1.Time the planting of nurseries to ensure that the blast season has passed before the seedlings reach the susceptible stage. This will involve planting

well-developed prenursery seedlings early in the rainy season, and ensuring their rapid growth.

2.Pay particular attention to irrigation during the short dry season and make sure that polybags have a sufficient, though not excessive water supply through- out the nursery period.

3.Where Recilia mica is prevalent, spray out host grasses in the vicinity of the nursery, and apply aldicarb monthly.

De Franqueville et al. (1991) tested several insecticides, and found that omethoate was also effective. Clones dif- fer in their susceptibility to blast (IRHO, 1992b), but breeding for resistance would not be a sensible approach to an easily controlled nursery disease.

12.1.5 Adult palm leaf diseases and disorders

12.1.5.1 Crown disease

Distribution: The disease was most prevalent in the Far East, particularly in the early Deli plantations. All but the most severe cases normally recover during the second year after planting, and effects on yield are then not serious.

Symptoms: A palm suffering severely from crown dis- ease has many of its leaves bent downwards in the middle of the rachis; at this point the leaflets are absent, or small and ragged (Plate XVA). These symptoms originate in the spear leaf, where the folded leaflets begin to show a rot of their edges or centre (Kovachich, 1957). This rot spreads throughout the central portion of the leaf so that when the leaf unfolds the leaflets of this section are disintegrating or already missing. The rachis bends at the point where the leaflets are absent. In severe cases all the leaves surrounding the spear may be bent down, and the spear itself may have a rot of its terminal por- tion which turns brown and hangs down. Under these extreme circumstances, crown disease may have a severe effect on early development and yields. The dis- ease normally affects palms in the second to fourth year in the field, but instances have been reported in the nursery and up to 10 years of age.

Causes: No pathogen has been identified, and it was assumed in the early days that the disorder was physio- logical and might be inherited. The latter assumption proved correct (see below); with regard to the former it was suggested that palms suffering from the disease have low leaf magnesium contents and that the incidence of the disease might be affected by magnesium and potas- sium manuring (Hasselo, 1959). Breure and Soebagyo

Diseases and Pests of the Oil Palm

 

 

 

 

 

 

397

 

 

 

 

 

 

 

 

Table 12.2 Reduction in yield of palms with crown disease

 

 

 

 

 

 

 

 

 

 

 

 

 

Class

Severity of

Palms in

 

Yield loss/palm (%)

Yield loss/ha (%)

 

 

symptoms

class (%)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1st 6 m

7–38 m

1st 6 m

7–38 m

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

No symptoms

80.8

0

0

 

0

0

 

1

Slight

8.0

15.1

2.7

 

1.2

0.2

 

2

Mild

6.3

22.8

3.4

 

1.4

0.2

 

3

Severe

4.5

36.1

4.5

 

1.6

0.2

 

4

Very severe

0.25

54.3

34.0

 

1.4

0.8

 

5

Extremely severe

0.12

43.2

27.1

 

0.5

0.3

 

 

 

Total loss/ha (%)

 

 

 

 

 

6.1

1.7

 

 

 

 

 

 

 

 

 

 

 

 

From Dumortier (1998).

Loss per hectare  loss/palm  palms in class.

(1991) compared two sites, and observed lower leaf boron levels (and rather lower magnesium) at the site with more severe incidence of crown disease.

Thompson (1934) stated that the ‘decreased rigidity’ of the rachis was due to insufficient lignification of the parenchymatous tissue, and Monge et al. (1994) found that the fibres of vascular bundles in the rachis were very thin-walled in affected leaves. The leaves, how- ever, tend to be quite rigid, although bent. Monge et al. (1994) considered that crown disease and spear rot might be manifestations of the same disorder. Alvarado et al. (1997) also suggested the two were associated, but in their trials with a susceptible progeny, poor drainage reduced the level of crown disease, but increased spear rot incidence (see Section 12.1.7.2). Boron application also reduced crown disease incidence.

Effects on yield: Severity of crown disease varies between environments (Breure and Soebagyo, 1991). In Papua New Guinea (PNG), where incidence can be severe, Table 12.2 shows a yield loss of about 6% in the first 6 months of production, and just under 2% over the next 2.5 years. Breure and Soebagyo (1991) estimated losses of 4.5% over the first 3 years of production in North Sumatra, with the greatest loss in the first year.

Control: De Berchoux and Gascon (1963) showed that pure Deli progenies in the Ivory Coast were highly susceptible. La Mé material, free of crown disease, gave crosses with Delis which were also free of crown disease, but Congo material, which showed several cases of the disease, gave Deli  Congo crosses with a quarter to a half of the palms showing the disease. The authors postulated that susceptibility to crown disease is due to a single recessive gene. Some examples from their results are given in Table 12.3, showing close agreement with the expected segregations. Thus, it appeared practicable

to select palms that would not produce susceptible indi- viduals in their progeny; in particular, it would be valu- able to have pisifera shown to be homozygous for absence of crown disease (CC), as the Congo (Sibiti) palm S127P appeared to be.

Blaak (1970b) found that with some palms in Cameroon the expected inheritance occurred. However, other crosses gave segregations that could best be explained by the presence of an inhibitor gene which, when homozygous, suppressed expression of the dis- order. Examples from Blaak’s results are also given in Table 12.3.

If susceptibility is controlled by only one or two genes, then its elimination from a breeding programme should be easy, although Blaak (1970b) pointed out that the presence of an inhibitor gene complicates selection, since detection of a palm of cc genotype (susceptible) is only possible by test crossing with a palm that is known not to have the inhibiting gene. The losses noted by Dumortier (1998) are quite small, but 6% extra crop over the first 6 months of production is clearly worth having. However, Dumortier showed that pisifera DM742.207 transmitted crown disease suscep- tibility to its offspring, and yet it consistently gave the highest yielding progenies in most environments (Dumortier and Konimor, 1999). Thus, it is under- standable that some oil palm breeders have not regard- ed eliminating suceptibility from their programmes as being very important.

12.1.5.2 Leaf wither, Pestalotiopsis leaf spot or grey leaf blight

Distribution: A virulent type of leaf withering has been troublesome in parts of Colombia, Ecuador and

398

The Oil Palm

Table 12.3 Incidence of crown disease in various crosses in the Ivory Coast and Cameroon, and expected segregation with and without Blaak’s inhibitor gene; expectations shown are those if the inhibitor were dominant

Presumed

Cross

 

Observed (%)

Expected

 

 

Expected  inhibitor

genotypes

 

 

 

 

 

(no inhibitor) (%)

(%)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Without

With

Without

With

Without

With

 

 

 

 

 

 

 

 

 

 

de Berchoux and Gascon (1963)

 

 

 

 

 

 

 

 

 

cc  cc

D115D selfed

0.5

99.5

 

0

100

 

 

 

CC  cc

L10T  D115D

100

0

 

100

0

 

 

 

cc  CC

D115D  S127P

100

0

 

100

0

 

 

 

Cc  cc

L219T  D115D

57

43

 

50

50

 

 

 

Cc  cc

S7T  D115D

52

48

 

50

50

 

 

 

Cc  Cc

L219T  D10D

81

19

 

75

25

 

 

 

Cc  Cc

L239T  D128D

71

29

 

75

25

 

 

 

Blaak (1970b)

 

 

 

 

 

 

 

 

 

 

Cc ii  Cc ii

1.2229 selfed

79

21

 

75

25

 

75

25

Cc II  Cc ii

3.417  1.2229

77

23

 

75

25

 

75

25

Cc II  Cc II

3.417 selfed

100

0

 

75

25

 

100

0

Cc ii  Cc ii

5.37 selfed

76

24

 

75

25

 

75

25

Cc II  Cc ii

3.415  5.37

96

4

 

75

25

 

100

0

Cc II  Cc II

3.415 selfed

100

0

 

75

25

 

100

0

Cc Ii  Cc ii

15.4624  5.1295

85

15

 

75

25

 

87.5

12.5

 

 

 

 

 

 

 

 

 

 

 

From de Berchoux and Gascon (1963) and Blaak (1970b).

Honduras, and has caused much defoliation, with sig- nificant effects on yield. It is also commonly seen in Colombia on E. oleifera palms.

Symptoms: The first symptom is the appearance of small brown spots with yellowish haloes. These spots soon coalesce into brown necrotic areas which spread over the leaflet tissue and later become grey and brittle (Plate XIC). There is a sharp line between the brown and grey areas and in the latter a species of Pestalotiopsis is found, black specks indicating the location of spore- bearing acervuli (C.W.S. Hartley, 1974, unpubl.).

Cause: The disease has been described as Pestalotiopsis leaf spot and grey leaf blight in Malaysia, but there the fungus is only associated with old and near-moribund leaves and is not considered of economic importance (Turner, 1981). The severity of the attacks in Latin America seem to be due to the easy access given to the young leaves by the feeding activities of insects, but it is also possible that the strains of the Pestalotiopsis species involved are more aggressive.

Genty et al. (1975) showed that Leptopharsa gibbicarina

(Hemiptera: Tingidae) was the principal means of infecting young leaves. This insect punctures the leaflets alongside the midribs, producing whitish spots with their surrounds stained with excrement (Genty et al.,

1983). In Ecuador, Peleopoda arcanella has been impli- cated in assisting infection of the leaves by Pestalotiopsis sp. (Turner, 1981). Two species of Pestalotiopsis are the usual entrants, but species of Helminthosporium, Curvularia and other genera may also gain access to the leaflets.

Effects on yield: The disease has caused considerable defoliation on some plantations and, as would be expected, this has been followed by serious yield decline. Bunch production falling from 18–20 to 12–15 t fresh fruit bunch (FFB)/ha in adult areas, and from 11 to 7–8 t/ha in young plantings has been reported (Jiménez and Reyes, 1977).

Control: The attacks on moribund tissue in Africa and Malaysia have often been associated with magnesium deficiency symptoms (Bull, 1961a). In Colombia, the disease is less severe if potassium and magnesium are in balance (P.L. Gomez, pers. comm., 2001).

Control of L. gibbicarina by aerial spraying of propoxur, fenitrothion or phosphamidon has had consid- erable success in checking the disease, but more recently trunk injection or root absorption of monocrotophos has been advocated; as noted in Section 12.2.1.4, these methods of application have advantages in terms of selectivity. Monocrotophos injection has also been

Diseases and Pests of the Oil Palm

399

successful in Honduras (Vessey, 1981). The possibility of biological control has been studied (Genty et al., 1983; Guerrero, 1985), and Mendez (2000) described successful management of the disease by encouraging

Crematogaster ants and using Beauveria and Paecilomyces fungi to control the insect vector.

12.1.5.3 White stripe

Distribution: The condition is sporadic and, in Asia, is said to be more common on alluvial soils, particularly organic clays or mucks. Affected palms have reduced yield.

Symptoms: Narrow yellowish-white stripes are found on each side of the leaflet midrib and extending its whole length. The stripes are sharply divided from the adjoin- ing green (often dark green) tissue. Affected palms usu- ally recover, and Rajaratnam (1972b) reported that the chlorotic tissue might turn green after about 7 months and that the symptom was more prominent in young leaves than in old. He also showed that chlorosis was due to failure of the palisade mesophyll cells to elongate and that apparent recovery was through an increase in the chlorophyll content of the spongy mesophyll, and not through development of the palisade cells. Turner (1981) stated that symptoms are more severe in Malaysia than elsewhere, and that typically they appear at 2–3 years of age, becoming more severe at 3–5 years and then becoming chronic.

Effects on yield: Moderately affected palms yielded 15% less than healthy neighbours, and severely affected palms nearly 50% less (Rajaratnam, 1972b). However, Tohiruddin et al. (2002) found that yield was some- times positively correlated with white stripe incidence (see below).

Cause: It has been suggested that the disorder is of genetic origin. A certain tenera  dura cross showed similar percentages of white stripe when planted in Ivory Coast and in East Cameroon; also certain Deli selfs in Ivory Coast showed the symptoms while others did not (Ollagnier and Valverde, 1968; Gascon and Meunier, 1979). However, the general view is that the cause is nutritional: boron deficiency and a high leaf N:K ratio have both been suggested. In boron-deficient seedlings, Rajaratnam (1972a) found chlorotic patches, in which the palisade mesophyll had not developed, but the patches did not form stripes. In one trial, palms showed a degree of recovery when boron was applied, but in another they did not.

Turner and Bull (1967) considered that a nitrogen:potassium imbalance was the main cause of white stripe, but Rajaratnam (1972b) observed the

disorder in palms with N:K ratios well below that sug- gested as critical by Turner and Bull. Tohiruddin et al. (2002) found a positive correlation of white stripe inci- dence with N:K ratio in only one of five fertiliser trials studied. In that trial, there was a negative correlation with leaf K content, and a stronger correlation with rachis K content (rachis K is more sensitive to applied K than leaf content; see Section 11.4.2.2). In unfer- tilised palms in that trial, rachis K content was below 0.6%, much lower than in the other trials. The correla- tion of white stripe incidence and yield was negative and statistically significant.

In two other trials, where leaf N content exceeded 2.8%, white stripe incidence was positively correlated with leaf N content, and positively (but not signifi- cantly) correlated with yield. There were no correla- tions with leaf boron content. These results support the N:K imbalance hypothesis, but suggest that the leaf N:K ratio is not an adequate indicator of the imbalance.

Treatment: Turner (1981) suggested substantial applications of potash with reduction of nitrogen appli- cations, but Tohiruddin et al. (2002) distinguished between white stripe caused by low K status, and by high N status. Where rachis K is low, potassium fer- tiliser should be applied. Where leaf N exceeds 2.8%, reducing N input may reduce white stripe incidence, but it is also likely to reduce yield. In this situation, symptoms can be expected to disappear with time, since as palms grow older, leaf N content tends to fall, and K reserves increase.

12.1.5.4 Leaf mottle (mancha anular)

Distribution: This condition, which often leads to death of the palm, has been reported from Ecuador and Peru and is described by Turner (1981). It has been called ring spot, but that term is already used for a nur- sery disease.

Symptoms: When the spears open they fail to become fully green, and spots of pale tissue remain. These may be circular, or elongated and almost rectangular, and may form almost continuous streaks. Younger fronds then become chlorotic. This leaf symptom is followed by the rotting of the root system and spear, although Turner (1981) considered the spear rot to be secondary. Developing bunches may also rot. Palms may die within 3 months of the first symptoms, but some palms continue to grow and yield for several years, despite showing leaf symptoms.

Cause: Martínez (1988) suggested that a virus might be the cause, but this has not been confirmed. Renard and de Franqueville (1989a) described nursery dry bud

400

The Oil Palm

rot as being similar (see Table 12.1). A similar disease of coconuts is transmitted by two species of Sogatella (Homoptera: Delphacidae) ( Julia and Mariau, 1982).

Treatment: Fungicides, insecticides and antibiotics have been tested without effect. Diseased palms tend to be scattered throughout a field, but incidence is much higher with dense grass cover than with a legume cover (Dzido et al., 1978). A good leguminous cover should therefore be maintained in areas subject to this condition.

12.1.5.5 Patch yellows

Distribution: This disease appears to be confined to Africa, where it is widely distributed, though sporadic and affecting only a small proportion of palms. A con- dition in Malaysia known as wither tip, from which both Fusarium oxysporum and F. solani have been isolated, was described by Turner (1981) who suggested that it was allied to patch yellows.

Symptoms: Infection takes place in the unopened spear leaf and for this reason the lesions at the sites of infection appear opposite each other on the leaflets when the leaf opens (Plate XIB). The lesions are circular or oval with rings of pale yellow, sometimes with brown centres. The patches may appear all along the lamina. Later, the centres of the patches dry out and drop away, giving a typical ‘shot-hole’ appearance, or, if the patches are towards the edge of the leaflets, a raggedly indented appearance. The purely yellow patches persist and darken, and can be seen to have small orange spots within them (Bull, 1954).

Cause: Wardlaw (1946a) reported that, following the discovery of F. oxysporum associated with vascular wilt disease, a second strain of F. oxysporum which closely resembled the first had been shown to be associated with patch yellows in Congo. Kovachich (1956a) later proved its pathogenicity. Prendergast (1963) found that a patch yellows strain of F. oxysporum did not cause vascular wilt in the nursery test (see Section 12.1.6.2 below).

Control: The disease affects up to 1.8% of palms in the areas where it is found, and evidence for genetic susceptibility was provided in Kovachich’s pathogeni- city tests. In Nigeria, a small proportion of families were susceptible (A.G. Prendergast, pers. comm., 2001). Kovachich suggested selection to eliminate susceptible lines, but the disease does not seem to be sufficiently serious to justify this.

12.1.5.6 Minor leaf diseases

The oil palm leaf is susceptible to patchy discoloration and necrosis from a variety of minor pathogens, some

of which are listed in Table 12.1, and to surface covering by epiphytic and saprophytic organisms. These often cause the older leaves to appear far from healthy, but as the oldest leaves contribute relatively little photosyn- thetically, the effects may be small.

Black ‘sooty mould’ is often found to grow on the older leaves of adult palms and occasionally spreads over a large proportion of the leaf surface, giving the palms a blackish-grey appearance. Sooty moulds usually grow as epiphytes on honeydew, the exudate of partially digested plant sap from plant lice, but it is not certain whether this is always the case in oil palm (Wood, 1968a).

Several of the most common fungi to be found in Africa as constituents of the epiphytic flora are listed in Turner’s Micro-organisms associated with oil palm (1971). Among these, the Ascomycetes Apiospora sp., Meliolinella elaeidis and Meliola elaeis may be mentioned. Meliolinella elaeidis is recorded as also being found in America (Costa Rica) on E. oleifera. Epiphytic flora may appear on the upper or lower surface of the leaves. In West Africa the black mould usually found on the upper surface consists of discrete circles of about 5 mm diameter; on the lower surface the black mould is in irregular patches of less dense material. In Malaysia, sooty moulds of Brooksia,

Ceramothyrium and Chaetothyrium spp. develop on insect secretions on the leaves (Williams, 1965; Turner, 1981).

Brooksia tropicalis is common in Africa.

Lichens are often found among the epiphytic flora on oil palm leaves, forming small grey–green encrust- ations on the upper surface of the leaflets (Turner, 1971).

12.1.6 Stem and root diseases

Root and stem diseases are characterised by fracture and drying out of fully developed leaves, leaving the spear leaf and some surrounding leaves standing erect. These early symptoms may be accompanied by a change in colour, drying out or wilting of one of the more erect younger leaves; bud and spear rots tend to be characterised by symptoms in the centre of the crown. The spear leaf may be directly affected or the surrounding leaves show a sudden chlorosis. Suc- cessive spear leaves may be shortened, have peculiar ‘little leaf ’ formations, or cease to develop, leaving a palm with an empty centre. These general symptom differ- ences between stem and root diseases on the one hand, and bud and spear rots on the others give a rough guide when deaths occur or alarming disease symptoms appear, but dissection of the palm must follow to deter- mine exactly where the site of destruction is. The site of decay with the root and stem diseases is the bole, trunk

Diseases and Pests of the Oil Palm

401

or roots, the disease killing the palm by denial of water and nutrients to the crown. Bud and spear rots, in con- trast, kill the palm by growing towards and reaching the single growing point.

12.1.6.1 Dry basal rot

Distribution: This disease appears to be confined to West Africa and, although the pathogen is a common soil inhabitant, the disease was not discovered in epi- demic form until 1960. One estate in Nigeria was dev- astated and thereafter minor outbreaks occurred in several parts of Nigeria, West Cameroon and Ghana. In the first, epidemic deaths were common, but recovery then became more usual and further serious outbreaks have not been reported.

Symptoms: The foliar symptoms, which usually appear at the end of the dry season, are preceded by extensive bunch and inflorescence rot. The rachis of certain leaves then becomes fractured submedianly, although the leaflets remain green for a considerable period before they eventually die (Plate 12.2A). Occasionally, a young leaf high up in the centre of the crown becomes necrotic and dries out, and this precedes the necrosis of the older leaves. It is quite common for a complete ring of leaves to exhibit the submedian fracture while the upper leaves are still erect, and this gives the newly affected palm its char- acteristic appearance. Later, the upper leaves and the spear will be similarly affected and the palm dies, or it may make a recovery at any stage. A palm that survives may take several years to come back into bearing. Attack has usually been on palms that have recently come into bearing, but 10-year-old palms have also been affected.

The characteristic internal symptom of the disease is a dry rot at the base of the trunk (Plate 12.2B). This rot is well established by the time the primary leaf symp- toms are apparent. In the transition zone between rot- ted and healthy material many vascular bundles are necrotic, and it is possible to trace infection from an infected root or leaf base into the base of the trunk.

Cause: The cause of dry basal rot was shown by Robertson (1962a,c) to be the ascomycete Ceratocystis paradoxa, the imperfect stage of which is known as Thielaviopsis paradoxa. The latter has been implicated in fatal yellowing (Section 12.1.7.2). Ceratocystis para- doxa is a soil inhabitant widely distributed throughout the tropics of Africa and Asia, and causes diseases of several other crops. Its sudden appearance in West Africa as the cause of a serious condition was unexpect- ed and gave rise to investigations on conditions con- ducive to its spread. An epidemic at Akwukwu in Nigeria occurred on acid sands soils with an unusually

Plate 12.2 Dry basal rot, Ceratocystis paradoxa. (A) A severely infected palm showing sub-median fracture of the lower leaves. (B) A palm showing external symptoms of the disease, dissected to expose the dry rot at the base of the trunk.

low clay content at depth (15–17% at 2 m), and minor outbreaks at the Nigerian Institute for Oil Palm Research (NIFOR) Main Station also occurred on

402

The Oil Palm

fields with little clay in the profile. This led to the belief that incidence might be connected with soil–climate relationships. A further outbreak at NIFOR in 1967 followed a severe dry season. Incidence varied between fields from 0.1 to 10%.

Two features of the spread of the disease are import- ant. In the outbreak at Akwukwu many deaths occurred in the first 2 years, amounting to about 30% in one area. Thereafter, very few deaths occurred and there was con- siderable recovery; although new infections occurred, these did not give rise to many further deaths (Hartley, 1988). All the palms that recovered were bearing bunches by 3 years after the last survey (Rajagopalan, 1965). The other feature of this disease at the NIFOR Main Station was that incidence in one field of 9-year- old palms was mainly confined to progenies having the same female parent. In Robertson’s pathogenicity tests he found he could infect all seedlings through a root dip- ping technique; nevertheless, inoculated progeny lines planted in the nursery showed marked differences in dis- ease incidence (Robertson, 1962b). Selection for resist- ance is therefore a promising line for the future should the disease once more become important.

12.1.6.2 Fusarium wilt or vascular wilt

Since its description by Wardlaw (1946b) in Congo, Fusarium wilt has been considered one of the most men- acing of oil palm diseases. The disease has been observed on plantations in Congo, Nigeria and West Cameroon, in the Ivory Coast, and elsewhere in West Africa. It has also been recorded in Brazil and Ecuador. Its effects are serious: in the acute form the palm rapidly dies, while chronically affected palms yield little or nothing.

Symptoms: In the more usual, chronic form of the disease in mature palms, the older leaves become desic- cated and the rachis breaks near the base or at some dis- tance from the base, the ends of the leaves hanging downwards. This feature has been used to distinguish the disease from Ganoderma, in which the leaves col- lapse at the base and closely cloak the stem. The disease usually proceeds gradually along several leaf spirals, with younger leaves becoming successively affected. The erect and still green leaves in the crown become successively more reduced in size and are often chlor- otic, and the palm may stay in this state for several years before the crown eventually collapses.

Occasionally, a mature palm suffers a rapid death through an acute attack. The leaves dry out and die rapidly while still in an erect position, and then snap off about 1 m or more from the trunk, usually during strong winds (Plate XIIB). The remaining

leaves die quickly. All stages between the acute and chronic forms are encountered.

Symptoms of the disease in young palms in which no trunk has yet been formed are somewhat different. In these palms the ‘lemon frond’ symptom is frequent: a leaf somewhere in the upper middle part of the crown (fourth to 15th leaf ) develops a bright lemon-yellow colour before drying out from the tip to the base. Leaves at about the same level then turn yellow and dry out, to be followed by some of the younger leaves, which will die while many of the older ones remain green. Newly devel- oped leaves become successively smaller (Plate XIIA), and death of the whole palm usually takes less than a year. It should be noted that the striking lemon-frond symp- tom is not always seen, and in southern Congo a general yellowing of the leaves before death was more usual.

Internal symptoms of the disease are quite distinct- ive (Plates XII, XIII). The vascular bundles are nor- mally pale yellow or whitish, but when diseased they become brownish-grey or black, and a cross-section of the trunk therefore shows a speckled appearance (compare Plates XIIID, E). Discoloration, which is associated with the presence of gum, is confined to the xylem vessels (Plate XIIE); blackened fibre strands do not indicate vascular wilt. Such blackening often occurs in older palms and sometimes in other condi- tions, and the inexperienced observer can therefore be misled into a wrong field diagnosis. Moureau (1952) pointed out that although discoloration of the xylem ves- sels is normal in palms over 20 years old, in these palms the blackening decreases towards the top of the palm instead of becoming accentuated as in the case of wilt. In palms with acute wilt, A.G. Prendergast (pers. comm., 2001) found large cavities, often more than 30 cm across, in the stem within 2 m below the crown; these cavities were filled with a dense mass of Fusarium mycelium.

Drying-up of the leaves and death of the palm are caused partly by the destruction of the roots and partly by the blocking of the xylem vessels by gels and gums. Diseased vessels may at first occur in only one section of the stem base, and this probably accounts for leaf symptoms being confined at first to certain spirals. Xylem vessels in the centre and at the top of the stem then become diseased and the symptoms spread across the stem, so that a large proportion of the vessels at the top are affected. In young palms, up to 6 years old, dis- eased vessels are usually widely dispersed throughout the base.

The disease is normally recognised in the field by the external symptoms, but Mepsted et al. (1991) showed that the internal symptom of vascular browning could be detected non-destructively by taking tissue samples

Diseases and Pests of the Oil Palm

403

with an auger (Plates XIIC, D). Using this method, they found that 25% of a sample of palms, classed as healthy by external appearance, showed internal symptoms of the disease (latent infection). Buchanan (1999) used the auger method, and found a poor correlation between external and internal symptoms. In one palm family, 54% of apparently healthy palms showed internal symptoms, while, conversely, 40% of palms with exter- nal symptoms showed no internal browning. Buchanan noted that simultaneous infection by Cercospora and Ganoderma may produce external symptoms somewhat similar to those of Fusarium wilt, so erroneous identifi- cation of wilted palms may partly explain his results.

Palms may recover, and recovery has become increas- ingly common as plantings of tolerant material are extended (Renard et al., 1991).

Cause: Fusarium oxysporum f.sp. elaeidis (abbreviated below to Foe). One isolate from Congo was found to be F. oxysporum var. redolens (Ho et al., 1985). The pathogen is soil borne and it usually enters the palm through the roots, growing along the stele, which becomes blackened. Infection can take place through wounds in the stem base and through uninjured roots (Kovachich, 1948). Renard (1970) considered that entry of the mycelium was much impeded by lignification even with wounding and that rapid infection was mainly through the trans- mission of spores in the vascular system. Locke (1972), working with seedlings, showed that the pathogen is confined to the conducting elements of the xylem (Plate XIIIB) and can reach the stele from the tip of a lateral root or the damaged cortical tissue of a pneu- mathode. From the roots the mycelium penetrates into the xylem vessels of the vascular strands, where conidia and chlamydospores are also found. Locke considered that the plant had little defence against serious infection in spite of resin formation and tyloses (Plate XIIIC). However, Paul (1995) indicated that in resistant geno- types, gels and tyloses were produced earlier and accu- mulated to higher levels, and fungal colonisation was restricted compared with susceptible genotypes. In the latter, production of gels and tyloses was delayed and the fungus rapidly colonised the host.

Distribution: The first recording of the disease was briefly described by Wardlaw (1950b) as follows. ‘Dur- ing a visit to the Belgian Congo in 1946 I observed a wilt disease of the oil palm (Elaeis guineensis), and isol- ated Fusarium oxysporum from the necrosed vascular strands. In 1947, Messrs. S. de Blank and F. Ferguson, in a private report, announced the presence of this dis- ease in Nigeria and submitted cultures to me for iden- tification; and in 1948 I was able to confirm their diagnosis during a visit to the affected plantations.’

Pathogenicity was confirmed by Fraselle (1951). Thereafter, Fusarium wilt was found on several planta- tions in Nigeria and West Cameroon, in the Ivory Coast, and elsewhere in West and west Central Africa. The dis- ease has been recorded in Brazil (van de Lande, 1983) and Ecuador (Renard and de Franqueville, 1989b), and Flood et al. (1989) confirmed the pathogenicity of a Brazilian isolate by inoculation of clonal plants. Dossa and Boisson (1991) showed that, while Foe strains from Africa were in many different ‘vegetative compatibility’ groups, strains from Brazil and Ecuador were in the same group as strains from Benin and the Ivory Coast. This close rela- tionship was confirmed with restriction fragment length polymorphism (RFLP) markers (IRHO, 1992b). It is thus likely that the disease was introduced to South America from Benin or the Ivory Coast, probably on seed, as it has been shown that spores of Foe can be spread on seed (Locke and Colhoun, 1973; Flood et al., 1990).

Incidence and spread of the disease: In southern Congo, the greatest devastation occurred in replants. In West Africa the disease was largely confined to plantations, particularly replants, and for a long time was not much noticed in the groves. However, Aderungboye (1982) found that it was widespread in the drier Ogun and Ondo states of Nigeria, but infrequent or absent in the high-rainfall areas of the south-east. Oritsejafor (1989) found that the average incidence in Nigeria was 0.77% in palm groves and 1.35% in plantations. The disease is less frequent in plantations on forest land than on for- mer savannah (de Franqueville, 1991); in the latter, the disease usually takes the chronic form, whereas after forest the acute form is more common.

In plantations in southern Congo, and in Nigeria in replants, wilt is commonly found in young palms that have recently come into bearing. However, in West Africa the disease has also attacked older palms that have been in production for 10 years or more (Prendergast, 1957). Renard and de Franqueville (1989b) indicated that dis- ease development depends on the previous history of the site. In new plantings, the first cases may not be seen until 6–10 years after planting, but in replants in previ- ously infected areas, losses may occur within a year of planting. De Franqueville and Renard (1988) found that wilt incidence in a replant was correlated, not with total losses in the old stand, but with the percentage of previously infected palms which were still living at the time of replanting.

Prendergast (1957) stated that healthy, vigorous palms in good soil suffered little from the disease and he showed that, in areas of potassium deficiency, incidence was sub- stantially reduced by the application of potassium fer- tiliser.This finding was confirmed in experiments in both

404

 

 

 

 

 

The Oil Palm

 

 

 

 

 

 

Table 12.4 Yield of palms with missing neighbours at Binga, Congo. The main cause of vacancies was

 

 

 

death from Fusarium wilt

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Missing neighbours

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

1

2

3

4

 

 

 

 

 

 

 

 

 

 

Yield (kg FFB/palm) for palms without symptoms

93.0

98.5

104.5

77.9

15.7

 

 

Yield as % of that of palms without missing neighbours

100

106

112

84

17

 

 

Number of palms in class

400

179

23

8

1

 

 

 

 

 

 

 

 

 

 

From Dumortier et al. (1992).

the Ivory Coast and Benin (Ollagnier and Renard, 1976). In the nursery test, in contrast, Prendergast (1963) found that nitrogen reduced susceptibility, but potassium had no effect. Ho et al. (1985) found that drought stress increased the severity of symptoms in seedlings.

Prendergast (1957) showed that diseased palms occurred in pairs more frequently than would be expected by chance, indicating infectious spread between neighbouring palms. Dumortier et al. (1992) found that palms with missing neighbours, in an area where the main cause of death was Fusarium wilt, were more likely to have wilt themselves than those without missing neighbours: of 1600 palms without missing neighbours, 17% had wilt, compared with 24% of 1000 palms with one or more neighbours missing. Only 18 palms had three neighbours missing, but 35% of those had wilt. It has generally been assumed that the disease is soil borne, but Moureau (1952) mentioned aerial spread by spores, and Cooper et al. (1989) showed that the pathogen sporulates profusely on male inflorescences, and thus could be spread by spores.

Effects on yield: Acute wilt kills the palm, but Prendergast (1957) observed that infection rates as high as 20% had no apparent effect on yield, and suggested that this was due to yield compensation by palms adja- cent to vacant points. However, Dumortier et al. (1992) compared the yields of apparently healthy palms, with and without missing neighbours, and found that, although yield did increase slightly in palms with one or two neighbours missing (Table 12.4), the increase was not sufficient to compensate for the missing palms. When more than two neighbours were missing, yield was depressed (although the number of palms with more than two neighbours missing was small). If this depression is real, an explanation could be that palms with several neighbours missing are themselves infected (see above). Yield in the year before a palm died from acute wilt was only 54% of that of healthy palms, while palms with chronic wilt gave a yield less than 30% of that of healthy palms. The effects of deliberate and sys- tematic thinning on yield are discussed in Chapter 9,

where it is concluded that, at the planting densities most widely used, the yield increase from palms adjacent to gaps is unlikely to be sufficient to compensate for the palms removed. This is even more likely to be true for the patchy thinning that would result from disease. Thus, it appears likely that any incidence of acute wilt will reduce yield.

With chronic wilt, palms remain alive and may recover. Renard et al. (1993) looked at the effects of chronic wilt on the yields of four classes of palms:

•healthy palms

•palms with typical chronic wilt symptoms

•palms which had had the disease but had recovered

•palms with internal signs of infection (browning of the vascular tissue) but no external symptoms, described as ‘latent infection’.

They estimated yield losses due to the disease as 15% in a susceptible cross and 6% in a tolerant cross (Table 12.5). The difference was attributable to lower yields from, and a larger number of, recovered palms in the susceptible cross compared with the tolerant cross. In both crosses, palms with latent infection yielded much the same as healthy palms. Although healthy trees of the susceptible cross gave a slightly greater yield, overall yield was greater from the tolerant cross.

Physiology of diseased palms: In nursery seedlings, the main symptoms are a reduction in petiole length and leaf area. Mepsted et al. (1995a) observed symptoms of water stress in infected nursery palms (closed stomata, lower leaf water potential, greater resistance to water flow from stem to leaf ), but they considered that water stress was not the cause of stunted leaf development, because stressed but uninfected plants did not show stunted growth. Application of a gibberellin inhibitor, paclobutrazol, caused stunting symptoms similar to those of Fusarium wilt. Application of gibberellic acid to infected palms restored petiole length to normal, but had no effect on leaf area. The authors concluded that wilt symptoms might be due, at least in part, to an upset in gibberellin metabolism.

Diseases and Pests of the Oil Palm

 

 

 

405

 

 

 

 

 

 

 

Table 12.5 Effects of chronic Fusarium wilt on yield.

 

 

 

 

 

The weighted mean yield is (yield as % healthy)  (palms in class)

 

 

 

 

 

 

 

 

 

 

 

Class of palm

Yield

Yield as

Palms in

Contribution

 

 

 

(kg/ha per year)

% healthy

class (%)

to yield (tFFB/ha.yr)

 

 

 

 

 

 

(143 palms/ha)

 

 

 

 

 

 

 

 

Cross L2T  D115D (tolerant)

 

 

 

 

 

1.

Healthy

111.8

100

75

11.99

 

2.

Chronic wilt

35.6

32

3

0.15

 

3.

Recovered from wilt

90.8

81

13

1.69

 

4.

Latent wilt

107.4

96

8

1.23

 

5.

Dead

0

0

2

0

 

 

 

 

Weighted mean: 94

 

Total: 15.06

 

Cross L2T  D10D (susceptible)

 

 

 

 

 

1.

Healthy

121.4

100

61

10.60

 

2.

Chronic wilt

26.4

22

4

0.15

 

3.

Recovered from wilt

79.4

65

27

3.07

 

4.

Latent wilt

113.4

93

6

0.97

 

5.

Dead

0

0

2

0

 

 

 

 

Weighted mean: 85

 

Total: 14.79

 

 

 

 

 

 

 

 

From Renard et al. (1993).

Chronic and acute wilt: Mepsted et al. (1995b) found that water stress symptoms in nursery palms were much more severe in the older leaves, opened before the palms became infected, than in the younger, stunted leaves. They suggested that stunting might be an adaptation by the palm to reduce the water stress caused by occlusion of the xylem vessels, and speculated that, in field palms, failure to adapt in this manner might result in the acute form of wilt, while in palms that were able, or had time, to adapt, the chronic form would result. Prendergast (1963) and de Franqueville (1991) found no obvious dif- ference in behaviour in the nursery test between Foe strains isolated from acutely infected palms, and from palms with chronic wilt. This suggests that the differ- ence between forms of the disease may be in the host reaction. According to de Franqueville (1991), however, the acute form is more frequent in plantations on forest land than on former savannah. Such a difference is more easily explained in terms of pathogen strains than by host reaction, but differences in climate could be a factor.

Control: In Congo, the destruction by fire of all dis- eased palms and their neighbours was recommended and the replanting of areas where Fusarium wilt had been prevalent was discouraged (Moureau, 1952). In the Ivory Coast, de Franqueville and Renard (1988) stated that all trees infected with chronic wilt must be removed at least 5 years before replanting. However, they were unsure how early such removal should start, and sug- gested as a compromise the removal of any infected palms not producing at least one bunch per year. Calapogonium

caeruleum or Pueraria as cover increases wilt incidence, and Renard and Quillec (1983) recommended planting grass species such as Brachiaria instead, suggesting that competition for nitrogen between the grass and the palm discouraged Fusarium infection. Competitive grasses may themselves depress palm yield, though, and the recommendation of de Franqueville and Renard (1988) to keep a strip of bare soil on either side of the row of young palms seems preferable. Turner (1981) considered that any effect of fungicides was likely to be short lived. Renard and de Franqueville (1991) found a significant increase in wilt in plots mulched with EFB, and a reduction when potassium fertiliser was applied.

Breeding for resistance: The most promising method of control is by the breeding of resistant lines. Prendergast (1963) was the first to develop a technique for the screening of seedlings for tolerance to the disease at the nursery stage (Plate XIIIA); his method was adopted with little modification by Renard et al. (1972). The method involves inoculation by pouring a suspension of Foe spores onto the bulb of the seedlings or the exposed roots around the collar. Prendergast (1963) described the symptoms in nursery seedlings in detail, and showed that results were not much affected by the size of the seedlings at the start of the test. He also noted the importance of the inoculum level and the time of evalu- ation: with too light an inoculum, or too early evaluation, few cases might be observed, and differences would be hard to detect. However, if the inoculum was very heavy or evaluation was late, all plants might die.

406

The Oil Palm

The standard nursery test requires large numbers of plants of each family, and hence large areas of nursery space; Prendergast (1963) used 40 seedlings per family, and Renard et al. (1991) up to 160. Locke and Colhoun (1974) developed a method of inoculating very young seedlings grown in compost with two known levels of inoculum, and then compared their growth with that of seedlings grown in uncontaminated compost. Deter- minations were made of the number of propagules in the soil so that subsequent inoculations could be related to normal soil levels. The fungus was recovered from progenies showing both large and small reductions either in weight per plant or in ‘leaf area product’. It was demonstrated that some progenies were tolerant of infection in lightly contaminated compost only, and some in both lightly and heavily contaminated compost; others showed high susceptibility at both levels. A high degree of repeatability was attained, but this method does not seem to have been adopted by oil palm breeders.

Flood et al. (1989), using clonal plants, were able to reduce the numbers required to only 12 plants per clone, by using inocula with a known, constant concen- tration of fungal spores, and by detailed classification of the severity of symptoms on each plant.

Sound statistical analysis of the data from nursery inoculation trials is essential, if reliable results are to be obtained (reviewed by Porter, 1989). In all trials, the percentage of infected plants in each family has been recorded. Prendergast (1963) then divided the progen- ies in a trial approximately into quartiles, or classified as resistant those that differed from the mean by at least one standard error. Renard et al. (1972) calculated a ‘wilt index’, as the percentage of wilt-infected plants in a progeny divided by the mean wilt percentage of all the progenies in the trial. The best method appears to be that described by de Franqueville (1984). A wilt index was calculated in the same way as by Renard et al., but after angular transformation of percentages for individ- ual plots, data were statistically analysed, and progenies were only accepted as resistant if they had significantly lower losses than either the mean of the trial, or stan- dard crosses of known performance.

An alternative to the standard nursery test was devel- oped by Mepsted et al. (1995c). This involved inocula- tion of 2.5 cm sections from near the tip of the rachis, by immersion in a suspension of Foe spores under mild vac- uum. Within 8 days, rachis sections from susceptible clones turned completely brown internally, whereas those from resistant clones showed little or no brown- ing. This test is much faster than the nursery test, which takes several months. In addition to speed, the method has the great advantage that it can be applied to individ-

ual palms. The nursery test is based on the average per- formance of a group of palms, so the only way to screen individuals is as clones, or by studying the progeny of a self-pollination. Subsequent experience has shown that the rachis test only works well on palms that are in good health; nutrient-deficient palms, or palms infected with Cercospora elaidis, showed severe browning in both sus- ceptible and resistant clones (Buchanan, 1999).

The inheritance of resistance is discussed briefly in Section 5.3.5. Prendergast (1963) and de Franqueville (1984) considered that the most susceptible families could be quite consistently and repeatably identified by the nursery test. De Franqueville also found a reason- able correlation between results of the nursery test and disease incidence in the field, in a heavily infested part of Congo. Renard et al. (1972, 1980) also showed that tolerant seedlings in the nursery test give rise to palms with a low incidence of wilt in the field.

Porter (1989) described resistance breeding in Congo, where selection was based primarily on performance in disease-infested fields (Plate VID), backed up by the nursery test. He gave examples of parents consistently transmitting resistance or susceptibility to their offspring in field trials. For example, 14 crosses derived from palm 69MAB (see Fig. 5.4) were all more resistant than the trial mean, 12 of them significantly so. Conversely, of seven crosses from palm 2/5710 (see Fig. 5.5), six were more susceptible than the trial mean, five significantly so. Corley (1993) and de Franqueville et al. (1995) found dif- ferences between clones in susceptibility.

In Nigeria, Rajagopalan et al. (1978) found that, among 336 progenies, none was immune but 149 showed sufficient tolerance to be considered valuable for breed- ing; and certain pisiferas consistently gave tolerant crosses with a range of duras. Prendergast (1963) and Locke and Colhoun (1974) also observed that no prog- enies appeared to be wholly immune to the disease. However, Rosenquist et al. (1990) noted that pure Dumpy Deli dura material (see Section 5.1.1.2) appeared to be virtually immune: two families gave 0% and 1% wilt in the nursery test in Cameroon, while one family in Congo had suffered no losses after 10 years in the field, a figure recorded in only two other families out of more than 450 in the programme.

There are reports of resistant material from other countries proving susceptible when imported to Nigeria (Oritsejafor, 1989), but replicated trials were not involved. De Franqueville (1991) tested three strains of Foe on 66 different families. There were significant differences between the strains and between the families, but no strain  family interaction. Mepsted et al. (1994) tested three isolates of the fungus from different parts

Diseases and Pests of the Oil Palm

407

of Africa, on 14 clones. The isolates differed in aggres- siveness, but as in de Franqueville’s trial, the clone  isolate interaction was not significant. These two studies indicate that resistant material selected in one area should remain resistant when transplanted elsewhere.

Symptomless infection: Ho et al. (1985) isolated

F. oxysporum from roots of healthy palms in Malaysia; these strains were apparently non-pathogenic, causing no disease symptoms. Flood et al. (1989) showed that one such strain did cause mild wilt symptoms in a sus- ceptible clone, however.

Mepsted et al. (1988) found that inoculation of seed- ling roots with a non-pathogenic isolate could prevent subsequent infection by pathogenic strains. Diabate et al. (1992) confirmed this ‘cross-protection’ effect, and showed that phenolic compounds accumulated in the palm roots after inoculation, whether this was with a pathogenic or a non-pathogenic strain. Susceptibility appears not to be due to a lack of these phenolics, as both resistant and susceptible palms produced them, at similar levels, in response to the non-pathogenic strain of F. oxysporum. Presumably, in susceptible palms the fungus normally spreads more rapidly than the build- up of phenolics; preinoculation with the non-pathogenic strain may allow sufficient build-up, in advance of infec- tion, to confer resistance. Both preformed and induced antifungals were extracted from xylem fluids and petiole tissue by Mepsted et al. (1995c), with the effect being particularly pronounced in resistant material. Paul (1995) suggested that preformed antifungal compounds were also present in oil palm roots, but the identity of the compounds in roots or petioles was not determined.

Diabate et al. (1992) suggested that there might also be competition between pathogenic and non-pathogenic strains in the soil. Flood et al. (1989) had previously suggested that competition for an ecological niche may be the reason that Fusarium wilt is not present in Malaysia: any strains accidentally introduced from West Africa would face competition from native Malaysian strains.

Plant quarantine: The demonstration that spores of Foe can be carried on oil palm seeds (Locke and Colhoun, 1973), and even on the kernel surface inside the shell (Flood et al., 1990), poses potential problems for plant quarantine. Flood et al. (1994) showed that the stand- ard 40°C heat treatment used to break dormancy (see Chapter 7) greatly reduced the level of infection, but some viable spores remained. They developed a method of fungicide application involving vacuum infiltration, which eliminated spores, including any within the shell. This treatment should be applied whenever seeds are exported from areas where wilt occurs.

Spores of F. oxysporum were also found in batches of freeze-dried pollen used for oil palm breeding, and were shown to be pathogenic (Flood et al., 1990). This contamination can be detected by plating out samples on a Fusarium-selective culture medium, but this is laborious if many samples are involved, and a method of decontamination would be useful.

Conclusion: There is no doubt that Fusarium wilt can be devastating, and there are instances of plantations in Africa being abandoned or converted to other crops because of the disease. Given the large quantities of seed and pollen exported from Africa in the past, it is sur- prising that the Far East has remained free of the dis- ease. Perhaps symptomless infection by other strains of F. oxysporum gives a degree of cross-protection, but the very strict quarantine measures now enforced by Malaysia and other countries appear sensible (Section 12.1.10). Breeding of resistant material is clearly the best approach to controlling the disease, and as a precaution, breeders in the Far East would be wise to gather information on the performance of their mater- ials in areas where the disease occurs.

12.1.6.3 Ganoderma trunk rot or basal stem rot

In the more severely affected areas in the Far East, over 50% of palms may succumb to Ganoderma. For many years, Ganoderma was regarded as a disease of old palms, of little economic importance because such palms would soon be replanted. In the mid-1950s, however, the disease started to attack much younger palms in the Far East, particularly in areas planted after coconuts or replanted from oil palms (Turner, 1981). In recent years, the dis- ease has been the subject of much research in Malaysia and Indonesia. Most of this work was summarised in Flood et al. (2000a), which included a good general review of the current status of the disease (Ariffin et al., 2000). Gurmit (1991) gave a useful review of research up to that date.

Symptoms: The usual first symptom of infection by Ganoderma is similar to that of drought conditions: a failure of the young leaves to open, so that a number of fully elongated but unopened ‘spears’ is seen in the cen- tre of the crown (Turner, 1966a). This indicates that the stem is already extensively damaged, so that water uptake is restricted; it is apparently a direct response to water shortage, so is not necessarily diagnostic for Ganoderma (Turner, 1966a). In old palms the lower leaves collapse, hanging vertically downwards from the point of attachment to the trunk (Plate XIVC). This is followed by the drooping of younger leaves, which turn a pale olive

408

The Oil Palm

green or yellowish colour and die back from the tip. Later, the base of the stem blackens, gum may be exuded and the distinctive fructifications of Ganoderma sp. appear (Plate XIVA). The whole crown of the palm may then fall off, or the trunk may collapse (Plate XIVB).

Bull (1954) described and illustrated the internal symptoms of old palms exhibiting Ganoderma trunk rot. In brief, it was found that the peripheral tissues are hard and unaffected by the rot, the black fibres in this zone being normal. Within the stem at the base of the palm the majority of the tissue is yellow-coloured and breaks up easily; mycelium can be found extending through the tissue. Roots are also found to be infected, the cortex being brown and decaying, the stele black. Large num- bers of sporophores may be formed, the early ones being small and rounded, the later ones being typical brackets.

Cause: Ganoderma species. Early work referred to G. lucidum, but that is a temperate species (Steyaert, 1967). The general consensus now appears to be that G. boninense is the main species pathogenic to the oil palm, at least in South-east Asia (Moncalvo, 2000). Idris et al. (2001) considered that two other species,

G. miniatocinctum and G. zonatum, were also important. Khairudin (1990b) suggested that G. tornatum might be involved, but Idris et al. regarded that species as a non-pathogenic saprophyte. In this chapter we will fol- low most authors in referring to the pathogen simply as

Ganoderma.

Navaratnam (1961) confirmed pathogenicity by inocu- lating both roots and stems of 40-year-old palms with Ganoderma mycelium. More recently, palms at the nursery stage have also been successfully inoculated (Amiruddin, 1993; Sariah et al., 1994; Ariffin et al., 1995b), and the pathogen has been reisolated (Khairudin et al., 1993).

Distribution: In Malaysia, the disease is much more prevalent on low-lying alluvial soils, particularly the coastal clays, than on inland soils, and it is on the former that the most serious attacks on young palms have occurred. Turner (1965a), examining attacks on young palms, showed that incidence on areas where the pre- ceding crop was coconuts was much higher than where planting followed forest or rubber (see also Fig. 12.1). Turner quoted two instances where fields of 15-year- old oil palms after rubber had 4 and 2% Ganoderma attack while adjoining areas which followed coconuts had incidences of 39 and 35%. The greatest losses were said to be where old coconut trunks had been buried, to pre- vent infestation by Oryctes rhinoceros (Section 12.2.4.1). Turner (1981) considered oil palm tissue to be a less conducive medium for the fungus than coconut tissue; he quoted survey results showing 24–28% infection in

 

70

 

 

 

Forest

(%)

60

Rubber

50

Oil palm

Coconut

incidence

40

 

 

 

Ganoderma

30

 

20

 

10

 

 

 

0

 

 

 

 

 

 

 

 

 

 

1

3

5

7

9

11

13

15

17

19

21

 

 

 

Years after planting

 

 

 

 

Fig. 12.1 Incidence of Ganoderma in oil palm in relation to previous crop. (From data of Gurmit, 1991.)

15-year-old palm to oil palm replants, but 6–81% in oil palms following coconuts (Turner, 1965b). However, Gurmit (1991) quoted a 15-year-old replant with 67% infection, and Fig. 12.1 suggests little difference in the rate of disease development in plantings after oil palm or coconut. On peat soils, a high incidence may be observed whatever the previous crop (Ariffin et al., 1990).

The effect of soil type on disease incidence is not yet understood. Gurmit (1991) suggested that the usually high soil moisture of the coastal soils might favour Ganoderma over other, antagonistic soil fungi. According to Swinburne et al. (1998), three types of Ganoderma can be isolated from oil palms in Malaysia; these were subsequently identified as different species by Idris et al. (2001). Ganoderma boninense (type A), was sig- nificantly more aggressive than type B (actually two species, G. miniatocinctum and G. zonatum), while

G. tornatum (type C) appeared to be a non-pathogenic saprophyte. Isolates from coastal soils, usually with a history of previous coconut or oil palm planting, were predominantly G. boninense, while those from inland soils were type B. This is consistent with the greater incidence on coastal soils, but it remains unclear whether it is simply coincidental that these types are associated with particular soils. Some live coconut palms were found to contain G. boninense as a symptomless endo- phyte, which could explain the high incidence in plant- ings following coconuts. Idris et al. (2001) speculated that infection by G. tornatum might offer some protec- tion against the pathogenic species.

Spread of the disease: If palms in the early stages of the disease are dissected, infection usually appears to have started from the roots. This, and the demonstration that

Diseases and Pests of the Oil Palm

409

neighbouring palms were often infected by the same strain of the fungus (Turner, 1965c), led to the assump- tion that infection under natural conditions is mainly by root contact with an infected palm orother inoculum source. Turner (1981) considered that Ganoderma was a weak parasite, and that it needed to develop saprophyt- ically in large masses of dead palm tissue before it had sufficient ‘inoculum potential’ to infect live plants. More recently, though, Hasan and Turner (1998) showed that even isolated roots from diseased trees could be a suffi- cient inoculum source.

Seedlings planted very close to Ganoderma-infected stumps may show disease symptoms within 1 or 2 years (Hasan and Turner, 1998). These authors showed that poisoned stumps gave higher infection, presumably because they rotted more quickly, and could be invaded by seedling roots more easily. Short (20 cm) stumps gave higher infection rates than 50 cm stumps, perhaps again because they rotted more quickly. Diseased stumps had largely ceased to be sources of infection 2 years after felling. Stumps of healthy palms colonised by Ganoderma became sources of infection, and rotted, more slowly than diseased stumps. Hasan and Turner suggested that they therefore remained infectious for longer. They also noted that infected palms may remain apparently healthy for long periods, before the internal damage becomes so severe that external symptoms develop. Ariffin et al. (1995a) found, by extraction of trunk samples, that in a 22-year-old planting, between 13 and 17% of palms classified as healthy were actually infected.

Most attempts at control have been based on the assumption that infection is by root contact with an infected palm or other inoculum source. Work with molecular markers has confirmed that this assumption is sometimes correct, as the same genotype was detected in diseased stumps as in seedlings planted close to the stump (Flood et al., 2000b). Some studies on spatial patterns of the disease have shown that diseased palms in some fields tend to be in clumps, which also indicates spread through root contact. However, in other areas the disease appears to be more randomly distributed, as might be expected if spread were by spores (Flood et al., 1998). Recent work with molecular markers (Miller et al., 2000) and mating compatibility studies (Miller et al., 1999; Pilotti and Sanderson, 2001) have indicated that a range of genotypes may exist within quite a small area. Even a single palm may be infected by more than one genotype of the pathogen. This sug- gests that basidiospores (which are sexually produced and thus genetically variable) may be an important mode of spread.

Bridge et al. (2001) used a Ganoderma-specific molec- ular marker to show the presence of Ganoderma, in young palms without external symptoms, in the pruned leaf bases. This indicates that the cut leaf bases provide a site for infection by spores. It is possible that the fungus then develops very slowly in palm tissue, and it may be a decade before external symptoms appear, followed by the production of sporophores and a new generation of spores. These may again infect cut leaf bases, this time causing upper stem rot (Section 12.1.6.5).

It seems likely, therefore, that the disease can be spread in two ways. Root contact with an inoculum source, such as old oil palm or coconut trunks, would result in early infection, within a few years after plant- ing. Outbreaks of the disease 15 or 20 years after planting may often be from aerial spores, although slow development of infection that took place more than a decade earlier is a possibility. In PNG it was concluded that spores had so far been the only source of the dis- ease (Pilotti and Sanderson, 2001).

Predisposing factors: The effects of previous crop and of soil type have already been mentioned. The very strong influence of soil type does not appear to have received the attention it merits, but several studies of the effects of fertilisers have been made. Akbar et al. (1971) indicated that nitrogen and magnesium may have some role in combating the disease, but more recent tri- als have given equivocal results. Potassium chloride and urea application have both increased disease incidence in some trials, and decreased it in others (Gurmit, 1991). Tayeb Dolmat and Hamdan (1999) found similar conflicting results with phosphorus and potassium in three trials, two on peat and one on a coastal alluvial soil. Gurmit (1991) noted that high soil salinity and low soil pH appeared to discourage the disease.

Effects on yield: Yield reduction may occur both from death of palms, and from reduced yield in infected but still living palms. Disease losses might be partly com- pensated for if palms next to gaps gave increased yield, and Turner (1981) stated that there was circumstantial evidence that losses of up to 20% might be compen- sated for. This may have referred to Prendergast’s com- ments on Fusarium wilt (Prendergast, 1957), but other work on that disease suggests that any yield increase is insufficient to compensate for the lost palms (Section 12.1.6.2). More recently, Hasan and Turner (1994) stated that yield compensation ceased at around 10% losses, but again presented no supporting data. The extent to which compensation occurs will depend on the optimal planting density, which varies with soil fer- tility and with type of planting material (see Section 9.3.3.1). In Sumatra, in plantings from the late 1960s

410

The Oil Palm

 

25.0

 

 

 

 

 

 

 

22.5

 

 

 

 

 

 

 

20.0

 

 

 

 

 

 

per year)

17.5

 

 

 

 

 

 

15.0

 

 

 

 

1965–74 ex-forest

ha

(a)

 

 

 

 

 

 

FFB/

 

 

 

 

 

 

30

 

 

 

 

 

 

(tons

 

 

 

 

 

 

 

 

 

 

 

 

 

Yield

25

 

 

 

 

 

 

 

 

 

 

 

 

 

 

20

 

 

 

 

1975–84 ex-rubber

 

 

 

 

 

 

 

 

15

 

 

 

 

 

 

 

80

90

100

110

120

130

140

(b)Surviving palms (per ha)

Fig. 12.2 Yield of individual fields in North Sumatra, with different amounts of Ganoderma losses. (From London Sumatra, unpubl.)

and early 1970s, there was little decline in yield until the surviving stand had fallen to about 115 palm/ha (Fig. 12.2a), but in more recent plantings, any loss of palms was associated with a loss of yield (Fig. 12.2b).

Khairudin (1995) found that yield of infected palms was reduced by 20–40% in the year before infection was detected. Effects may not always be so large, though: Nazeeb et al. (2000) showed that palms with Ganoderma yielded between 13 and 21% less than healthy palms at the same age. Gurmit (1991) compared the yield of a field badly affected by Ganoderma with that of a less infected field of the same age. The heavily infected field yielded 26% less at 11 years after planting, and 46% less at 15 years, by which time incidence was 67%. It was not stated what proportion of diseased palms had died.

Control at time of replanting: It is generally accepted that incidence of Ganoderma increases from one gener- ation of oil palms to the next, but there are few published data to support this contention, the evidence being mainly anecdotal. The 25-year time interval between generations makes reliable comparisons within the same field difficult, and possible genetic differences in sus- ceptibility would further confound comparisons (see

Breeding for resistance, below).

Table 12.6 shows data from three estates in Indonesia; there appears to be an increase in disease incidence

Table 12.6 Ganoderma incidence (% of palms severely affected or dead) in different generations, planted in different blocks at the same time

Estate

Age

 

Generation

 

 

 

(years)

 

 

 

 

 

 

1

2

3

4

 

 

 

 

 

 

 

 

1

11–15

1.5

7.8

17.9

4.7

2

11–15

 

–

10.6

10.2

–

 

16–20

 

–

14.9

17.5

–

 

21–25

5.1

9.3

17.7

–

 

25

11.4

8.6

–

–

3

11–15

 

–

11.1

11.1

12.4

 

16–20

20.5

20.6

17.8

9.4

 

21–25

 

–

27.6

21.3

–

 

 

 

 

 

 

 

All blocks were replanted by felling and windrowing; all disease scoring was done in 1999–2000 (de Franqueville, 2000).

over the first three generations in estate 1, and perhaps in estate 2, but not in estate 3. These data are from different fields, recorded at the same time, so could be confounded by the possible effects of soil type already mentioned above. As further evidence against a build-up from one generation to the next, Gurmit (1991) found that fields that ranged from 40 to 60% infection before replanting all had the same level of disease 9 years after replanting. The high incidence in first replants, as shown in Fig. 12.1, suggests that a build-up could occur, but there is little evidence for a further increase after the second replant. Despite this, most control measures are aimed at preventing such a build-up, on the assump- tion that spread is by root contact with an inoculum source.

The recommended method of reducing Ganoderma incidence has been to deal with it at replanting time, by ridding the fields of as much oil palm tissue as possible although, as will be seen, the effectiveness of this is unclear. Mechanical methods of ‘clean clearing’ were outlined by Turner (1981). Stimpson and Rasmussen (1973) gave an account of a system used on the coastal clays of Malaysia which entailed burning or, if this was not possible, cutting up, splitting the boles and windrowing the old oil palms so that they rotted rapidly. The method included prior poisoning of the palms and subsequent root raking and ploughing to bring up and dispose of pieces of palm base and other material that might form a focus for Ganoderma. The operations are costly, but have been regarded as essential in coastal areas. In inland areas the incidence of Ganoderma is not usually so great, and the emphasis on clean clearing has been less strong.

Diseases and Pests of the Oil Palm

 

 

 

 

 

411

 

 

 

 

 

 

 

 

Table 12.7 Effects of replanting method on Ganoderma incidence

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Location

Age when

Disease incidence and deaths (%)

 

 

Ref.

 

 

surveyed

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(years)

Underplanted

Felled and

Clean cleared

 

 

 

 

 

 

 

windrowed

 

 

 

 

 

 

 

 

 

 

 

 

Inland (Johore)

17

5.7

5.0

–

1

 

Inland (Johore)

20

5.5

4.5

–

2

 

Alluvial (Sabah)

13

23.4

24.2

–

2

 

Alluvial (Selangor)

15

33.0

17.6

14.0

 

3

 

 

Alluvial (Perak)

15

 

–

5.4

0.5

 

4

 

 

 

 

 

 

 

 

 

 

 

References: 1: Loh and Rajaratnam (1977); 2: Pamol Plantations Bhd (unpubl.); 3: Khairudin (1990a); 4: Gurmit (1991).

Table 12.8 Ganoderma incidence in first and second replants of the same blocks

Block

First replanta

 

 

 

2nd replantb

 

 

 

(2nd generation of palms)

(3rd generation of palms)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Year planted

Ganoderma (%)

 

Year planted

Ganoderma (%)

 

 

 

 

 

 

 

 

1

1955

4.8

(10 years)

1978

2.8

(10 years)

2

1954

19.5

(12 years)

1976

21.6

(12 years)

3

1955

33.8

(9 years)

1975

32.4

(9 years)

4

1957

2.0

(9 years)

1980

3.5

(9 years)

Mean

 

 

15.0

 

 

 

 

15.1

 

 

 

 

 

 

 

 

 

 

 

From Gurmit (1991).

Method: aFelled and windrowed, stumps left in situ; b clean cleared by burning, and raking to remove root and bole tissue

Table 12.7 shows results of a number of comparisons of replanting methods. Disease incidence may be higher after underplanting, but in three out of four comparisons the difference was small. In two trials, the difference between felling and windrowing, and the clean clearing advocated by Turner (1981) and others, was also quite small.

Table 12.8 shows a comparison of first replants (the second generation of palms) with second replants (third generation). The first replant was done by felling and windrowing, without removal of stumps; the sec- ond replants of the same blocks were done by clean clearing. The almost identical figures in successive gen- erations can be interpreted in several ways.

•Gurmit (1991) concluded that, because clean clear- ing had not reduced the incidence, the method needed improving.

•If a large increase from one generation to the next is expected, it could be argued that clean clearing has

been effective in preventing such a build-up (but the evidence for such a build-up is equivocal; see Table 12.6).

•Comparison of clean clearing and windrowing in the same generation shows little advantage for clean clearing (Table 12.7), so the similar incidence in the two generations in Table 12.8 could, alternatively, be interpreted as evidence against a build-up from one generation to the next.

There is a danger of arguing in a circle, but consid- ering all the results available, we suggest that:

•Clean clearing only reduces disease incidence by a small amount, if at all (although what is meant by clean clearing may differ from one organisation to another).

•Incidence does not increase much from one gener- ation to the next, but is more dependent on the strain of Ganoderma prevalent in the area, the soil type or some other predisposing factor.

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The Oil Palm

Hasan and Turner (1994) found that very few seed- lings planted over 1.5 m away from infected stumps became infected. One approach to replanting severely Ganoderma-infected fields could therefore be to plant down the centre of the interlines (4.5 m from the old stumps). It would probably take the roots of the seed- ling more than 2 years to reach the stumps, by which time, provided they were poisoned, they should have rotted enough as to be no longer infectious. Hasan and Turner (1994) also suggested that a 2-year fallow period should precede replanting, but this would only be acceptable from the financial viewpoint if some other crop could be taken. Gurmit (1991) showed that 1 year under soya beans before replanting had no effect on subsequent disease incidence.

If spores are the main mode of spread, rather than vegetative contact (see Spread of disease, above) then clean clearing may make little difference. The import- ant factor would be a source of spores; for instance, infected old palms adjacent to the young palm area.

Nazeeb et al. (2000) suggested planting at a higher than normal density when a high incidence of disease is anticipated. They presented yield data up to the seventh year of harvesting to support this suggestion, but by that stage deaths were only 3–7%. Wood (1999) noted that an underplanting with 80% Ganoderma losses by 20 years was more profitable, on a discounted cash-flow basis, than clean clearing, even if the latter completely prevented losses. This is because yield many years after the investment has relatively little value in a discounted cash flow.

Other methods of control: Various attempts to control the disease with systemic fungicides have been made. Early work was not successful (Loh, 1977; Jollands, 1983), perhaps because massive lesions may already be

present by the time external symptoms are seen. Ariffin and Idris (1993) found that trunk injection of dazomet had some effect. George et al. (1996) obtained a signifi- cant reduction in incidence from a combination of car- boxin and quintozene, which were more effective than dazomet. The effect was probably to retard develop- ment in infected palms, rather than to prevent infec- tion. Effects on yield were not quoted.

Biological control by inoculating lesions with cultures of various micro-organisms was suggested byVarghese et al. (1976), and subsequent work was reviewed by Sariah and Zakaria (2000). Soepena et al. (2000) described the use of conidia and chlamydospores of Trichoderma koningii as a ‘biofungicide’, but presented no data on its effects. Sariah and Zakaria (2000) studied the effects of Trichoderma harzianum, alone and in factorial combina- tion with a mycorrhizal preparation, dried palm oil mill effluent and calcium nitrate, on Ganoderma devel- opment in inoculated seedlings. Table 12.9 is based on their data, and suggests that Ca(NO3)2, dried effluent and mycorrhizal inoculation may all have had some effect, but Trichoderma did not. Yow and Jamaluddin Nasir (2001) claimed that young palms, inoculated with mycorrhiza in the nursery and planted next to diseased stumps, remained free of infection by Ganoderma for at least 3 years, whereas uninoculated plants were ‘mostly infected’.

Excision of large, discrete lesions (‘surgery’) has been practised (Turner, 1968), sometimes successfully, but it is expensive, and treated palms may later collapse. Surgery is less likely to work with young palms than with old palms, but it is with young palms, with a long life ahead of them, where the greater benefit would be obtained from a ‘cure’. Gurmit (1991) indicated that a back-hoe could be used to do the surgery mechanically, and gave

Table 12.9 Effect of various treatments on Ganoderma development in seedlings

Treatment

 

Foliar symptomsa

Roots with

Bole infection

 

 

(%)

lesions (%)

(%)

 

 

 

 

 

Calcium nitrate



18.9

0.0

0.0

 



41.0

27.6

23.8

Dried palm oil mill effluent



22.8

2.1

0.6

 



37.1

25.5

23.2

Mycorriza



26.0

3.4

1.1

 



33.9

24.2

22.8

Trichoderma harzianum



28.2

13.9

13.0

 



31.7

13.8

10.8

 

 

 

 

 

Four-month-old seedlings were inoculated with rubber-wood blocks. Data are main effects from a factorial design, from Sariah and Zakaria (2000).

a Percentage of leaves that were desiccated or chlorotic.

Diseases and Pests of the Oil Palm

413

figures for costs, but not for efficacy. Hasan and Turner (1994) obtained some benefit from surgery, in terms of better survival and yield 36 months after treatment.

Removal of diseased palms is probably more widely practised than surgery. Such palms are identified in reg- ular inspection rounds, and removed by poisoning, felling, cutting up the trunk and excavating the bole tis- sue to hasten decay. Gurmit (1991) presented data which showed that this approach only slightly reduced the rate of spread of the disease. Over a 4-year period, incidence increased from 13 to 50% where infected palms were removed, and from 12 to 57% without removal.

Mounding of the base of diseased palms with soil, after surgery, was suggested by Lim K.H. et al. (1995) (Plate XIVD). Numerous new roots developed from the trunk above the point of surgery, with treated palms suffering fewer deaths and some remission of symp- toms. Hasan and Turner (1994) found that surgery was unnecessary, mounding alone being just as effective. Ho and Khairudin (1997) found that mounding reduced the death rate from 34% to 2%, over 24 months after treatment. Yields were over 30% higher because of this. Treatment with dazomet as well as mounding gave only a small additional benefit, insufficient to cover the extra cost. By 36 months, only 14% of mounded palms had died, compared with 71% of untreated palms, and yield was over 50% greater as a result (Ho, 1998).

Sanderson and Pilotti (1997; also Sanderson et al., 2000) argued that the disease could be controlled by ensuring that fruiting bodies were never allowed to develop to the point where spores were released. Whether this is a valid approach will depend on the extent to which spread is by spores, rather than root contact. The method needs to be tested.

Breeding for resistance: This is an obvious approach, particularly given its success against Fusarium wilt (Section 12.1.6.2). Differences in incidence between West African and Deli material were observed in Indonesia (Akbar et al., 1971), and de Franqueville et al. (2001) showed significant differences between families in Ganoderma incidence in eight of 12 breeding trials, and between clones in six of seven clone trials. In one trial, for example, incidence in individual families ranged from 12 to 75%, 24 years after planting. An index was calculated, in the same way as for Fusarium resistance (Section 12.1.6.2), and showed reasonable consistency for the same parents in different crosses and trials. The development of inoculation methods for plants at the nursery stage should make resistance breeding easier, provided that the processes of infection in seedlings and in mature palms are similar. Amiruddin (1993) used several different in vitro inoculation methods to compare

three clones, and observed consistent differences among the clones in susceptibility, but Ariffin et al. (1995b) found no differences among 20 progenies in a nursery inoculation trial.

Conclusion: Ganoderma has been a serious problem in some areas in Malaysia and Indonesia for 40 years. Despite recent collaborative research (Flood et al., 2000a), our knowledge of the disease still depends heav- ily on anecdotal evidence, and several aspects remain surprisingly obscure. A great deal has been invested in clean clearing at replanting time, but there is little evi- dence that this has had much effect. Nor is there con- vincing evidence that the problem is getting worse; it still needs to be confirmed that the disease increases from one palm generation to the next. An attempt to collate the extensive data that must be available in plantation records, in relation both to palm generation and to replanting methods, might be very informative (Wood, 1999). More research on predisposing factors also appears to be needed.

Mounding of diseased palms, perhaps combined with fungicide treatment, can delay death and improve yield. This appears to be preferable to the removal of diseased palms, which does little to prevent disease development. Systematic removal of all fruiting bodies, to prevent spread by spores, has yet to be tested as a means of control. It appears to us that, in the longer term, the best approach to controlling the disease in areas where it is prevalent may be to develop tolerant material, using nursery inoculation for screening, in much the same way as has been done for Fusarium wilt.

12.1.6.4 Marchitez sorpresiva, sudden wither or ‘hartrot’

Distribution: This disease has been serious on plant- ations in Colombia, Ecuador and Peru (Plate XID). In Surinam the disease has been described as ‘hartrot’ (van Slobbe et al., 1978), and a similar condition has occurred in Bahia, Brazil.

Symptoms: The disease is characterised by a sudden rotting of all developing bunches, a reddish discoloration of the top of the petioles and rapid drying out of the leaves from the oldest ones upwards. This drying out is preceded by the appearance of reddish-brown streaks at the ends and centres of the lowest leaflets. The leaf then becomes successively pale green (as in nitrogen deficiency), yellow, reddish-brown and ash-grey (Genty, 1981). The palm dies in 2–3 weeks and as soon as the external symptoms appear the root system will be found to have rotted and to a large extent dried out. Similar symptoms, though proceeding at a slower rate,

414

The Oil Palm

are sometimes seen, and in Colombia this has been referred to as ‘marchitez progresiva’. However, there may be confusion with fatal yellowing (Corrado, 1970). In the typical marchitez symptoms, the spear is initially unaffected. The root rot is cortical. The cortex decom- poses in wet weather, but in the dry season tends to become necrotic and to detach itself from the stele. The rot starts to develop from the extremities and moves towards the trunk and towards the lower roots. The trunk itself usually remains healthy, but cases are reported where the base is rotted sufficiently to form a cavity (Martin, 1970; van den Hove, 1971). Palms have been attacked by marchitez from the age of 1 year.

Cause: Evidence has been accumulating that the cause is infection by a protozoan flagellate, Phytomonas staheli. These have been found in several countries in association with the disease, in the phloem of roots, meristem zone, spear base and inflorescence stalks (Dollet et al., 1977; Dollet and Lopez, 1978; Dzido et al., 1978), and are also present in certain weed species (Dollet, 1982).

A connection between infection and insect attack has been suggested. Lopez et al. (1975) considered that the root miner, Sagalassa valida (Genty, 1973) (Section 12.2.7.1) might be a carrier, although the disease is often absent in areas where the miner is present. In Colombia, where the disease devastated an area of palms growing in a heavy stand of Panicum maximum, the bug

Myndus crudus (Haplaxius pallidus) was found on palm leaves while the nymphs were present on the roots of P. maximum. The use of herbicides and insecticides reduced the incidence of marchitez (Mena Tascon et al., 1975), while inoculation experiments (Mena Tascon and Martinez-Lopez, 1977) also suggested that M. crudus might be playing a part in the transfer of the disease.

Desmier de Chenon (1984) and Perthuis et al. (1985) claimed that the bug Lincus lethifer, which lives in the axils of the leaves (Dollet et al., 1977), was the vector of the flagellate. Symptoms of the disease developed within a few months of the bug being released onto young palms, in the first instance at sites far from any cases of the disease, and in the second on a caged palm surrounded by other, healthy palms. It has also been shown that L. lethifer and another species, L. tumidifrons, can transmit the flagellates to coconut palms, where they cause hartrot (IRHO, 1992b).

Control: In view of the uncertainty of diagnosis it is difficult to recommend definite control measures. On the grounds that Sagalassa valida was likely to be play- ing a part in the transmission of the disease, applica- tions of insecticide around the base of the palms have been used to suppress the insect, and were strongly

recommended in Colombia, Ecuador and Peru (Lopez et al., 1975; Genty, 1977). As noted above, it now appears that Lincus spp., not S. valida, are probably involved, and Gomez et al. (1996) indicated that a combination of insecticide and herbicide (to eliminate alternate host plants for Lincus) has proved useful.

Cases of marchitez in E. oleifera  E. guineensis hybrids have not been recorded until recently, so it is possible that the planting of hybrids may be a method of avoiding the disease, although, as with fatal yellowing (Section 12.1.7.2), we doubt whether such a policy can be justified, because of the poor oil yield of hybrids. In Surinam, however, some hybrids have suffered from ‘hartrot’, although wild E. oleifera palms have not been observed with the disease (Alexander and Kastelein, 1983). Certain E. guineensis palms on a plantation devastated by this disease remained healthy (Hartley, 1988), so it may therefore be possible to select resistant progenies within the species.

12.1.6.5 Upper stem rot

Thompson (1937) described a lethal trunk rot that was serious only on deep peat and inland valley soils. The disease has appeared on other soils in both Malaysia and Indonesia, but is usually sporadic and not of major importance.

Symptoms: Typically, the lower leaves first become yellow and this symptom gradually extends to the mid- dle leaves and then to the spear. It is evident that spore infection of leaf bases takes place and that from these the fungus gains entry to the peripheral tissues of the stem. The brown decay appears to proceed slowly inwards from the leaf bases and in many cases a typical collapse of the stem at one point occurs, this usually following high winds (Plate XVB). The rot spreads upwards and down- wards in the stem, eventually killing the palm by invad- ing the crown. Two forms of fruiting bodies (normal and resupinate) appear, but only on palms where the leaf bases are extensively decayed. These are small, greyish- brown bodies with velvet-brown margins and are incon- spicuous among the leaf bases. The disease is confined to the stem and does not enter the roots.

Cause and distribution: Phellinus (Fomes) noxius; per- haps also Ganoderma. Thompson (1937) described the disease, but little work was done on it until Navaratnam and Chee (1965) and Turner (1969, 1981) gave accounts of its symptoms, incidence and control. The patho- genicity of P. noxius was proven by inoculation experi- ments. Ganoderma (Section 12.1.6.3) is also often found in association with upper stem rot, perhaps usually as a secondary infection, but Turner (1981) noted the

Diseases and Pests of the Oil Palm

415

possibility that this fungus might sometimes cause the disease, and isolations made in Sumatra have con- firmed the presence of Ganoderma in all cases of upper stem rot examined ( J. Flood, pers. comm., 2001).

Control: As there is usually much penetration of the stem by the time sporophores appear, it is desirable to detect the disease at an earlier stage. This can be done on palms of 10 years or older by striking the leaf bases with a wooden pole to detect the dull sound of an infected base. Incidence is insufficient to justify surveying palms below 10 years old. When the diseased leaf bases are cut away the extent of the infection can be explored. The lesion is excised from the stem with a harvesting chisel and the cut surfaces are treated with a preservative (Turner, 1969). Coal tar has been reported to give the best overall results. Treated palms give as high a yield as untreated palms, so the measures are considered worth- while wherever incidence is likely to be significant.

In a fertiliser experiment containing different progen- ies, there was evidence first that fertilisers containing potassium reduced incidence, and secondly that pro- geny differences in susceptibility existed (Navaratnam and Chee, 1965).

12.1.6.6 Red ring disease

Distribution: Red ring is confined to South America, where it has been found in oil palms in Venezuela, Surinam, Brazil and Colombia, where the similar disease of coconuts is prevalent. It has been studied on an estate in Brazil where it did considerable damage (Schuiling and Dinther, 1982). In unprotected areas, incidence can rapidly become high; Malaguti (1953) cited a group of 100 palms showing only 16 doubtful cases in January which by August had 22 deaths, nine doubtful or affected cases and only 69 palms remaining healthy.

Symptoms: The symptoms of this disease have been described by Malaguti (1953). The centre of the crown takes on a dwarfed appearance and the newly opened leaves become bundled together into an erect, compact mass, the leaflets being corrugated, twisted and some- times adhering to the rachis. Gum is exuded. Later, this crown of leaves turns slowly yellow and dries out, the rachis being a light brown colour with yellow spots. One or two of the intermediate leaves become bronzed and after 2–5 months all of the leaves gradually become yellow or bronzed, although remaining erect. Developing bunches rot and inflorescences fail to set fruit.

The most striking interior symptom is the brown cylindrical ring found in the trunk, 7–8 cm from the periphery and 1–2 cm broad (the ring is red in coconut, hence the name). This ring is most distinct towards the

Plate 12.3 Longitudinal (A) and transverse (B) sections of an oil palm suffering from ‘red ring’ in Venezuela.

base of the palm (Plate 12.3), but the infection proceeds upwards into the petioles and rachis of the leaves in the crown in which, on cross-sectioning, necrotic areas or spots can be found. This infection does not, however, invade the tissues of the stem apex or surrounding very young leaves.

Cause and spread: The coconut nematode, Rhadi- naphelenchus (formerly Bursaphelenchus or Aphelenchus) cocophilus, seems first to have been recorded on oil palms by Freeman (1925) in Trinidad. Proof that the nematode was the cause of red ring was obtained by Malaguti (1953), who did tests with inoculum from both the oil palm and the coconut. The disease appeared 2–10 months after inoculation.

Giblin-Davis et al. (1989) described the nematode as an obligate plant parasite, which is only able to repro- duce in palm tissue, but can also parasitise the weevil Rhynchophorus palmarum, which thus acts as an import- ant vector for the palm disease. Gerber and Giblin-Davis (1990) found that 90% of the weevils emerging from

416

The Oil Palm

infected palms carried the nematode either internally or externally. According to Gomez et al. (1996), Metamasius hemipterus is the main vector in Colombia, but Chinchilla et al. (1996) considered that the latter species was not a vector in Costa Rica, and Schuiling and Dinther (1982) found that M. hemipterus did not carry the nematode in Brazil. Warwick and Bezerra (1992) showed that trans- mission in coconuts could also occur by root contact, so this possibility cannot be ruled out for oil palm.

In Brazil, Schuiling and Dinther (1982) found both the nematode and R. palmarum on wild Oenocarpus distichus palms, and suggested that these may form a reservoir for infection of plantation palms.

Control: Incidence on an affected estate inVenezuela was greatly reduced by the taking of regular sanitary measures (Hartley, 1988, p. 629). Any diseased palm was poisoned, felled and burnt. The whole estate was inspected every 2 months for diseased palms. Most important was the protection of the palm against the type of wounding that provides sites for R. palmarum to lay its eggs. A high incidence of red ring in Brazil was preceded by very close leaf pruning which had resulted in wounding of the trunk (Schuiling and Dinther, 1982); care must be taken when removing leaves to make a clean cut sufficiently far up the petiole to avoid this. Circle-weeding with herbicides instead of with hand tools may also help to prevent wounding. Regular disinfection of tools has been suggested, together with treatment of the cut leaf and bunch-stalk surfaces, but provided that wounding is avoided, it is doubtful whether such precautions are necessary.

Oehlschlager et al. (1993) described a pheromone- based trapping method for the vector, R. palmarum. Chinchilla et al. (1995) showed that after a year of trap- ping, red ring incidence was reduced by two-thirds. Chinchilla et al. (1996) showed that pheromone trapping could also be used against M. hemipterus. Biological control of R. palmarum has been proposed (Moura et al., 1993).

12.1.7 Diseases of the bud or stem apex

Under this heading are grouped diseases occurring in the emerging spear and younger leaves inside the crown. Such diseases normally move towards the grow- ing point through the enclosed developing leaves of the ‘cabbage’, and when they reach it the palm is killed. Bud and spear rots have occurred widely in all three conti- nents and provide, perhaps, the most difficult problems of oil palm pathology. Investigation is difficult owing to the position of the transition zone, often in the heart of the palm, the rapid entry of secondary organisms into any rot within the cabbage and the multiplicity of con- fusing symptoms, some of which may be similar to those of deficiencies or genetic abnormalities.

Turner (1981) suggested that the term ‘spear rot’ should apply to diseases in which the primary rotting affects the spear, while ‘bud rot’ should be used only for diseases first destroying the unemerged leaves and the hidden base of the spear and also, usually, the apical meristem. The latter diseases are usually fatal, the former frequently not.

Plate 12.4 Young palm suffer- ing from spear rot, with no central leaves, Panama.

Diseases and Pests of the Oil Palm

417

12.1.7.1 Spear rot–little leaf disease

Distribution: This disease, previously called bud rot–little leaf, caused serious losses in the oil palm areas of southern Congo, where deaths exceeding 30% were recorded. Elsewhere in West Africa and in Asia cases rarely exceed a few per cent and are often confined to certain progenies; deaths occur but are rare.

Symptoms: The first sign of attack is a wet, brown rot on the lower part of the unopened spear leaf. Duff (1963) described how in very mild cases only the leaflets may be affected; the leaflet rot is passed from spear to spear until either it develops further or the palm grows out of the attack. Normally, however, the rachis becomes infected and the spear collapses and hangs down; it is not uncommon to find a spear leaf, in which the infected portion has rotted away altogether, lying on the ground where it has fallen. The spear rot grows downwards and may become a bud rot, but it is only if this reaches the growing point that the palm dies. In other cases, the first leaves to emerge after the spear rot are stumps consisting of the malformed basal portion of the rachis. Subsequent ‘little leaves’ are very short with a few corrugated shortened leaflets, but each successive leaf will be longer, and the leaflets less abnormal, until fully normal leaves are again produced. Little leaf is therefore a recovery symptom and does not precede rotting. Very similar symptoms may occur after damage by insects such as Oryctes (Section 12.2.4.1).

Cause: Many causes have been assigned to the ‘little leaf ’ symptom; for the early history of investigations, the paper by Bull and Robertson (1959) should be consulted. One common cause is boron deficiency (Ferwerda, 1954) (see Section 11.4.1), but this is not preceded by spear rot. Robertson (1960), working in Nigeria on palms of a susceptible progeny having regu- lar cycles of infection, showed that spear rot–little leaf disease was an active pathogen, since the appearance of little leaves and bud rot could be prevented by cutting off the spear below the rotted portion. Although prior insect attack is often suspected, it is not known for certain how spear infection takes place.

A bacterium of the genus Erwinia, similar to E. lathyri, was consistently isolated in Congo by Duff (1963) from young lesions and from tissue in advance of visible rotting, and inoculation experiments showed that spear rot–little leaf symptoms could be induced by it. Sus- ceptibility seems to be genetic, physiological and sea- sonal. In a field in Nigeria the disease was confined to one progeny. Genetic differences were also found in Congo, where there was an association between rate of

growth and disease incidence. The former was judged by the rate of elongation of spears and in susceptible palms the elongation rate fell below normal levels 2 or 3 weeks before an attack of the disease. It was believed that these circumstances, encountered in ‘unhealthy’ palms, allowed susceptible tissues to be exposed to infection for longer periods than normal. Palms in which the growth rate was artificially reduced by root or leaf cutting showed greater than normal susceptibility. In some instances there has been high incidence at either the beginning or end of the rains (Turner, 1981).

Kochu Babu (1988) described a similar disease from Kerala, southern India, where frequency of the disease increased with proximity to coconuts affected by root wilt or Areca palms affected by yellow leaf disease (Kochu Babu and Ramachandran, 1993). Mycoplasma-like organisms have been found in tissues of palms with these diseases, but were not found in oil palms with spear rot (Kochu Babu, 1988). Spear and bud rots in America have different and varying symptoms (Section 12.1.7.2).

Control: Duff (1963) provided growth and health records showing that the more vigorous progenies suffered less from the disease, and he inferred from this that anything interfering with vigorous growth increased susceptibility. While, therefore, the disease is not likely to be serious enough for control measures to be taken in areas where growth conditions, particularly those of water and nutrient supply, are good, in mar- ginal areas the planting of particularly vigorous pro- genies might be considered (Hartley, 1988).

12.1.7.2 Fatal yellowing or lethal bud rot (pudrición de cogollo, amarelecimento fatal)

A bud rot with variable symptoms, but not usually including the typical ‘little leaf ’ progression, has caused serious damage on plantations in Central and South America. Some plantations have been totally devastated, while others have suffered serious losses with many palms remaining in a moribund, unproduct- ive condition for long periods. There has been exten- sive research on this disease in recent years, much of it reviewed by Gomez et al. (2000). Turner (1981) called the disease fatal yellowing, from one of the characteristic symptoms. However, symptoms differ quite considerably between different areas, and the disease is not always fatal. There appear to be both ‘acute’ and ‘chronic’ forms, which may be different manifestations of the same dis- ease, as with Fusarium wilt (see above), but it appears more likely that more than one disease is being described under one name (IRHO, 1992b; de Franqueville, 2001) (see below, under Cause).

418

The Oil Palm

Symptoms: In the Llanos Orientales region of Colombia typical symptoms were described by Gomez et al. (2000) as dry or wet spear rot, accompanied by yellowing of young fronds (Plate XVD) appearing during wet periods, but disappearing during the dry season. The rot spreads downwards towards the growing point, but affected palms usually recover. Turner’s descrip- tion (1970) of the symptoms in La Arenosa plantation in northern Colombia was quite similar, but it appears that the disease killed many palms when the rot spread to the growing point. Turner noted a tendency for four to six young spear leaves to remain unopened and stuck together, as a ‘baton’, but he was not convinced that this was a valid disease symptom. When this symptom was seen, spear rot was said to follow within 10–30 days. The rot spread downwards, and within 1–9 weeks the spear collapsed, the rot reached the growing point and the palms died (Plate 12.4). This seems to be the typical ‘acute’ form of the disease.

As already noted, the symptoms and the severity of the disease appear to vary considerably from one country to another. De Franqueville (2001) described the dis- ease as having two phases: in the first phase, which may last for up to 12 years, increase is more or less linear, but then as foci start to develop, it moves into a phase of exponential increase. Table 12.10 lists the symptoms described by various authors. The main differences are in the extent of chlorosis, the speed with which the spear rot develops, and whether or not the rot reaches the growing point, causing death. The disease appears to take its most acute form in Ecuador, while the most extreme chronic form is that seen in Brazil (Para state) (Plate XVC) and Surinam. De Franqueville (2001) noted that more acute symptoms were seen elsewhere in Brazil. Swinburne (1990, 1993) reviewed symptoms in Brazil, Ecuador and Colombia; he noted that chloro- sis is a common response to stress in many plants, and is poor evidence for a common cause. The symptoms that he described for Brazil are similar to those for leaf mottle (Section 12.1.5.4); they are also similar to those described for iron deficiency by Setyobudi et al. (1998).

Cause: A wide range of causative agents has been suggested, including insects, fungi, bacteria and vir- oids. It should be noted that, as symptoms vary from country to country, it is not certain that they are of the same disease or have the same cause. Nieto (1992) believed that the Colombian and Brazilian forms were different diseases, as did Swinburne (1990, 1993). No direct evidence of infectious spread has been found, but in Surinam, van de Lande (1993a) and van de Lande and Zadoks (1999) found a tendency for the disease to spread with the prevailing wind. Downwind

spread was also noted in Brazil (Swinburne, 1990, 1993), but this observation was contradicted by Bergami Filho et al. (1998) and Laranjeira et al. (1998), who found, in later results from the same plantation as Swinburne, no preferential direction of spread, and a tendency for dis- eased palms to occur near water courses. These authors concluded that the disorder had an abiotic cause.

Ochoa and Bustamante (1974) isolated F. moniliforme var. subglutinans from diseased palms, and inoculation of palms grown in high humidity and low light inten- sity caused spear rot. This fungus was said to cause diseases with similar symptoms in sugar cane, maize, sorghum and Musa spp. However, Turner (1981) con- sidered that symptoms were unlike those normally associated with Fusarium attack.

Recently, it has been reported from Colombia that Thielaviopsis paradoxa causes the disease; drying and necrosis of the central leaves was induced by inoculation of seedlings (Gomez et al., 2000). These authors imply that T. paradoxa was also isolated from diseased palms in Ecuador and Brazil, with different isolates stated to have ‘different pathogenicities’ and with significant variation in the reaction of seedlings to different isolates. However, de Franquville (2001) stated that T. paradoxa was very rarely found in Brazil or Ecuador. It is possible that the chronic form of the disease seen in Colombia is caused by T. paradoxa, but that the acute form elsewhere has some other cause. It should be noted that the perfect stage of T. paradoxa, Ceratocystis paradoxa, is associated with dry basal rot (Section 12.1.6.1), which has quite different symptoms. The fungus is described by Turner (1981) as ‘one of the most common fungi recorded on the oil palm throughout the world’, and is not normally pathogenic. If this is the pathogen, there- fore, it is not yet clear what renders palms susceptible to it in Latin America, but not elsewhere.

Other fungi that have been isolated from diseased spears include Fusarium oxysporum and Botryodiplodia sp. in Colombia and F. solani and Sclerophoma sp. in Ecuador. Invasion of bud tissue by many species of bacteria follows the basal spear rot. Pathogenicity of these organisms has not been demonstrated, but de Franqueville (2001) speculated that a joint infection by a fungus and a bacterium may be involved. In Brazil, a viroid was suggested as the cause (Singh et al., 1988), but viroid-like RNA was found in both diseased and healthy palms (Beuther et al., 1992).

Insects have also been associated with the disease. In Colombia a Cephaloleia sp. was found to induce symp- toms similar to those of the early stages of the disease, and Urueta (1975) studied a range of other insects in diseased material. In Ecuador, Dzido et al. (1978)

Table 12.10 Symptoms of fatal yellowing in different countries

Symptoms

Colombia

Colombia

Ecuador

Surinam

Brazil

Panama

Nicaragua

Costa Rica

 

La Arenosa

Llanos Orientales

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

‘Baton’ effect

Often

No

No

–

No

–

–

–

Chlorosis (no. of leaves)

A few

A few, mild

A few

Few initially,

Many

Often none

Some

A few

 

 

 

 

later many

 

 

 

 

Spear break or collapse

Yes

Yes

Yes

Yes

Yes

–

–

Yes

Spear rot

Yes

Yes

Rapid

Usually

Eventually

Yes

Yes

Often

Leaves reduced in size

No

No

No

–

Yes

–

Yes

No

Spread to meristem

Rapid

Rare

Rapid

Slow

Slow

Slow

Slow

Rare

Bunch rot

No

No

–

No

Sometimes

No

–

No

Root rot

–

–

No

No

Yes

–

–

Malformation

Recovery or remission

Rare

Yes

No

Yes

Yes

Yes

Yes

Yes

Death

Yes

Rare

Yes

Yes

Eventually

–

Yes

Rare

Time from 1st symptom

4–5 months

Rare

1–2 months

1–2 years

1–3 years

–

–

Rare

to death

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Based on Turner (1970, 1981), van de Lande (1993b), van Slobbe (1986), Swinburne (1990, 1993), Chinchilla and Duran (1999) and personal observations (R.H.V. Corley). –: no information available.

420

The Oil Palm

found larvae of Alurnus humeralis (Coleptera: Chry- somelidae: Hispinae) and several other insects on dis- eased palms, but no definite connection between these insects and the disease has been established. Intensive efforts to identify vectors for the disease in Ecuador and Brazil have been unsuccessful (IRHO, 1992b; de Franqueville, 2001).

Wood (quoted by Swinburne, 1990, 1993) found that application of several different herbicides to the roots could cause symptoms similar to fatal yellowing. This suggests the possibility that, if diseased palms were poisoned as a control measure, transmission of herbi- cide to healthy neighbours may occur through root con- tact. This could be a contributory factor to the rapid spread of disease sometimes observed, as the neigh- bours would then show symptoms, and would be poi- soned in turn. Swinburne noted that this could not be the only cause, as fatal yellowing has been observed on smallholdings never treated with herbicides.

Predisposing factors: Whatever the pathogen, it is clear that there must be predisposing factors for the disease to develop. It has frequently been associated with poor drainage; Alvarado et al. (1997) found, in nursery trials with a susceptible progeny, that poor drainage led to significantly higher spear rot incidence than ‘excessive drainage’. However, drainage improve- ments did not slow the spread of disease in Ecuador (de Franqueville, 2001). Compacted soils and unbal- anced nutrition have been mentioned as predisposing factors: in northern Colombia, areas with compacted ex-pasture soils suffered the highest casualties. A low potassium/magnesium ratio was also suspected, but corrective manuring did not stop the disease spreading in La Arenosa (Hartley, 1965; Turner, 1981). Munevar et al. (2001) recorded higher incidence of the disease on soils with high clay content, on compacted soils and on poorly drained soils, and they found a much slower rate of spread of the disease where drainage was improved. De Franqueville (2001) mentioned several studies on trace elements, but noted that differences between dis- eased and healthy palms may be effects, not causes. In Colombia, leaf levels of phosphorus, copper and par- ticularly potassium were lower in infected than in healthy palms, while calcium and magnesium levels were higher (Munevar et al., 2001).

Chinchilla and Duran (1999) described a ‘dry spear rot’ in Costa Rica, which appears similar to the milder forms of fatal yellowing in Colombia. They noted particularly that the root systems of both healthy and diseased palms in affected areas were poorly developed, with many malformations (corky texture, abnormal branching), and that in affected palms vegetative vigour

was reduced before symptoms developed. In some areas recovery occurred after drainage and other manage- ment aspects were improved (see Control, below). They concluded that any pathogens were a secondary prob- lem, and that the disorder resulted from stress caused by poor soil aeration, in areas with a high water table and inadequate drainage, or suffering from soil com- paction or with shallow soil overlying gravel. However, de Franqueville (2001) quoted several studies in other countries showing that the root system of affected palms was normal and healthy.

Effects on yield: Very heavy losses of palms have occurred on certain plantations in Colombia, Ecuador and Brazil, with severe effects on yield. The heaviest losses have often been associated either with surgery to remove rotting spear tissue, which may expose the growing point and increase subsequent damage, or with deliberate destruction of diseased palms, and sometimes of healthy palms surrounding diseased points, in an effort to prevent the disease spreading. It appears not always to have been appreciated that, if 14% of palms show symptoms (whether fatal or not), and those palms and their six immediate neighbours are destroyed, close to 100% mortality will result. Van Slobbe (1988) noted that 46,000 palms had been ‘lost to the disease’ on one plantation in Brazil, but also that ‘all diseased palms have been eliminated within a month after detection’. De Franqueville (2001) later reported that this planta- tion had been abandoned because of disease losses. Van de Lande (1993b) recommended that even recovering palms should be destroyed in Surinam, as it was not certain that they were not still infectious.

In Colombia and Costa Rica, affected palms usually recover if they are left untouched (Chinchilla and Duran, 1999; Gomez et al., 2000). Santacruz et al. (2000) stated that 80–90% of palms recovered within 1.5–3 years. If palms are left to recover naturally, then yield also recovers, although this may take some time. Chinchilla and Duran (1999) stated that 2 years after the first symptoms had been seen, yields were still below those expected from palms of that age. Santacruz et al. (2000) found that yields of fruit were depressed by 30–40% in the first 2 years; oil/bunch was also slightly depressed. Acevedo et al. (2000) showed that both FFB yield and oil/bunch were reduced, and oil yield was halved in the worst affected palms.

Control: Speculative prophylactic applications of mixed fungicides and insecticides have not been suc- cessful (e.g. Gomez et al., 2000). Surgical removal of rotting spear tissues was widely practised in Colombia at one stage, but Santacruz et al. (2000) stated that recov- ery was just as good if no surgery was done. Destruction

Diseases and Pests of the Oil Palm

421

of infected or suspect palms has been widely practised but, as noted above, this has probably contributed more to the problem than to control.

Chinchilla and Duran (1999) indicated that the disorder could be prevented by improving drainage, mulching with composted empty bunches, and paying particular attention to potassium and phosphate nutri- tion. Affected blocks more than 20 years old were replanted, after subsoiling to rectify soil compaction. In Colombia, the problem has been greatly reduced where drainage was improved (Munevar et al., 2001). Deep ploughing, to improve soil aeration, and fertiliser application to lower the (Ca Mg)/K ratio, also reduced disease incidence (Acosta et al., 2002).

Breeding for resistance: It is possible that there may be resistant lines within E. guineensis: there are several reports of differences between progenies in disease incidence (e.g. Santacruz et al., 2000), with Deli  Avros material apparently being more susceptible than other origins in Colombia. In Ecuador, de Franqueville (2001) reported differences in rate of disease development between crosses of Deli  La Mé origin. Ayala (1999) described a method for testing the susceptibility of individual palms by inocu- lation of petiole sections with T. paradoxa.

On La Arenosa plantation in Colombia several plots of E. oleifera  E. guineensis hybrids survived while large numbers of the surrounding E. guineensis palms died. Similar observations have been made elsewhere (IRHO, 1992b). In areas subject to fatal yellowing, the planting of this hybrid has been regarded as a possible method of combating the disease, and large parts of La Arenosa were replanted with hybrids (Turner, 1981). However, oil yield of these hybrids is generally poor, because of low oil/bunch (although they may have other advan- tages; see Section 5.5.1.6 for further discussion).

Conclusion: In Ecuador and parts of Colombia, this complex of symptoms is undoubtedly a serious problem, but in many areas it seems that its importance has been exaggerated, and losses have been exacerbated by the measures taken to ‘control’ the ‘disease’. Good agro- nomic management appears to be the answer in most places. Planting of interspecific hybrids is unlikely to be justified, given the poor oil yield of such material, and we think that other reasons besides resistance to this disease are needed to justify the efforts put into breed- ing of such hybrids.

12.1.8 Diseases of the bunches and fruit

The occasional bunch and fruit rots that are encountered have not been extensively studied. Bunch-end rot has been associated with the Deli palm, particularly in

Malaysia (Thompson, 1934). Where neither lack of pollen nor insect attack is implicated, both this condition and complete bunch failure have been attributed to ‘over- bearing’ (Turner and Bull, 1967): the number of bunches is thought to be more than can be sustained by the palm’s processes of assimilation. As discussed in Section 4.3.6, however, the evidence for this is not convincing.

A bunch stalk rot has been connected with an unex- plained condition in West Africa known as leaf base wilt (Bull, 1954). The leaves bend down towards the ground and the stalks of bunches in the leaf axils also bend and may then begin to rot. The disease seems to be of purely mechanical origin and provided the rot is not so extensive that the bunch falls, the majority of fruit will develop. The small splits that appear in the stalk are invaded by a variety of saprophytic bacteria and fungi.

Marasmius palmivorus is common as a saprophyte on the cut leaf bases and on the decaying debris between these and the trunk. It appears that, under moist con- ditions in the Far East, sufficient inoculum potential may sometimes build up for healthy bunches to be invaded (Turner, 1965d). For a full discussion of the factors involved in the spread of the disease, Turner (1981) should be consulted. The obvious means of con- trol is to reduce, through sanitary measures, the medium on which the fungus grows on the palm. Rotting bunches should also be removed. Prophylactic spraying against Marasmius is not generally recommended, but Turner (1981) considered that on acid sulfate soils in Malaysia spraying may be economically justified.

12.1.9 Other abnormal conditions

The oil palm is subject to many abnormal conditions of growth and development, the causes of which are not known. Usually, although not always, these abnormal- ities are encountered where conditions are in some way adverse: impoverished sandy soils, long dry seasons, excessively wet conditions or intermittent waterlogging, grass competition, pockets of unusual soils, etc. In the more severe conditions bunch yield is usually negli- gible (Courtois, 1968). Only a few are mentioned here.

The term plant failure was used by Wardlaw for palms that almost ceased to grow. The rate of root and spear production, and the number of green leaves, decreases and the leaves that remain are erect and crowded. This, in turn, leads to a tapering of the trunk and progressive deterioration of the leaves, which are subject to various kinds of chloroses, dry out prema- turely and become brittle. There has been much specu- lation on the reasons for such palms being found dotted about among normal ones. The condition rarely occurs

422

The Oil Palm

in Asia. In Africa it is either considered to be of genetic origin, or may be associated with severe potassium and magnesium deficiency, or where soil depth varies sharply from point to point.

A condition known as choke, or dwarfed crown, has been encountered in fields in America suffering from red ring disease (Section 12.1.6.6), but does not appear to have the same cause (Malaguti, 1953). It has been referred to as hoja pequena (little leaf ), but the term ‘little leaf ’ should be reserved for the recovery symp- tom of spear rot–little leaf. In this condition all the leaves are smaller than normal, green, erect, bunched together and twisted with varying amounts of atrophy or corrugation of the leaflets. A sudden recovery from the condition is frequent, a tall cluster of normal new leaves being produced in the centre of the deformed ones, giving the palm a two-tier appearance. This type of deformity is not unknown elsewhere, and the term ‘choke’ has been used in Malaysia to describe a similar condition.

The oil palm is occasionally killed by lightning strike. Young palms can collapse rapidly and wither, but a sublethal condition known previously as rachis inter- nal browning is now also believed to be caused by light- ning (Turner, 1981). In older palms the trunk base is often charred. Lightning strike can usually be distin- guished from other causes of death because surround- ing palms show scorching on the side facing towards the strike.

Oil palms are quite tolerant of short-term flooding, and have been successfully established in river flood plains. However, if young palms are flooded to a level above the leaf axils, so that silt is deposited in the axils, extensive inflorescence abortion, and sometimes death of the palm, may follow. The risk diminishes as the palms grow taller, but significant losses have sometimes occurred in young plantations (Teoh et al., 2001).

12.1.10 Plant quarantine

The existence of serious diseases in some areas but not others justifies strict plant quarantine measures. Fungal spores are the most likely contaminant, but nematodes (unidentified, but in this case apparently harmless) have been found on germinated seeds (Kushairi and Rajanaidu, 2000). There are already examples of dis- eases spreading: as noted in Section 12.1.6.2, Fusarium wilt was spread from Africa to South America with oil palm seeds. The precautions adopted in Malaysia were described by Kang (1986), and included prohibitions on import of seed or pollen from areas where diseases of unknown aetiology occur, prohibition of import of

secondary hosts or insect vectors of disease, and limits on the quantities of seed imported. Precautions must start in the country of origin, with inspection of par- ents palms for disease and thorough cleaning of mater- ials. Where possible, seed and pollen should be screened for spores of important fungal diseases at an intermedi- ate quarantine centre, between the country of origin and the importing country.

Flood et al. (1994) developed a method of fungicide application involving vacuum infiltration, which elim- inated Fusarium spores from seeds, including any within the shell. This treatment should be applied whenever seeds are exported from areas where significant fungal diseases occur (see Section 7.1.5).

12.2 PESTS

The most important pests of oil palms are arthropods (insects, mites) and mammals, but other groups of animals may cause problems from time to time. In this section we have grouped the arthropod pests according to the damage that they do to the palm. We believe this arrangement will be more useful to the non-specialist than the conventional arrangement by zoological clas- sification. We have attempted to give the correct Latin names for pest species, but taxonomic changes are regrettably frequent. For some species we have also given well-known but no longer valid names.

Wherever possible, growers should use integrated pest management (IPM) systems. These involve the encouragement of biological control of pests, the adop- tion of agronomic methods that minimise the risk of pest outbreaks and, if pesticide application is unavoid- able, the use of selective chemicals and application methods with minimal side-effects. IPM has been widely applied in the oil palm industry for several decades, and much of the current understanding of the principles of IPM developed from work on tropical crops. As early as 1962, it was recognised that insecti- cides were causing pest attacks in oil palms, by upset- ting the ecological balance between the pest and its natural enemies (Wood, 1971). In Pests of oil palms in Malaysia and their control, Wood (1968a) enunciated the principles of what was then known as integrated pest control, with more than one-third of the book being devoted to explaining the reasons for pest out- breaks, methods of monitoring pest populations and ways of controlling pests without disruption of the natural balance in the agroecosystem (see also Wood, 1976c, 1987). As a result of the understanding of pest

Diseases and Pests of the Oil Palm

423

ecology that has been built up in all the main areas where the crop is grown, oil palms generally remain free of damaging pest outbreaks, without much need for intervention with insecticides.

Smith and Reynolds (1966) proposed an ecologically based classification of pests.

•Key pests are perennially occurring, and would cause severe damage in the absence of control measures. These are pests for which the limitation by natural enemies is generally inadequate.

•Occasional pests may cause sporadic economic damage, if the usually good environmental control, including biological control, is disrupted.

•Induced or potential pests cause no significant dam- age under current conditions, but have the potential to do so if environmental control were disrupted by changes in agricultural practice (usually the appli- cation of an insecticide).

This classification can be useful in considering how to manage a pest, although in practice it may be difficult to apply, as it refers to the natural balance between pests and their enemies, and not necessarily to that actually existing in the plantation.

There are no key pests of the oil palm, but a number of occasional pests can cause serious damage. For example, Wood et al. (1973) showed that a single bagworm out- break, causing more or less complete defoliation of 10-year-old palms, reduced yield by 40–50% over the next 2 years. Many factors may disrupt natural balance, and when an outbreak does occur, it is important to try to understand what caused it. This can be difficult, because outbreaks may persist for some time after the original disturbance has disappeared. Wood (1979) suggested that this was because it can take time for nat- ural enemy numbers to build up to match the numbers of pests. The more important occasional pests are dis- cussed later in this chapter.

There are numerous potential pests. For example, in South-east Asia Wood (1968a) mentioned over 80 arthro- pod species; in Latin America, Mexzón and Chinchilla (1993) listed 41 species, and Genty et al. (1978) over 70; in Africa, Mariau et al. (1981) listed 22 species, and Wood (1983a) 26 species. Some potential pests are listed in Table 12.11.

We have not given detailed recommendations on pesticides, as these are constantly changing, with new compounds being developed and older ones with- drawn. The most recent general recommendations appear to be those of Mariau (1993) and Jacquemard (1998).

12.2.1 Integrated pest management

The important aspects of an IPM system can be sum- marised as follows.

•Knowledge of the life cycle and ecology of the pest, and of its natural enemies, is required, if biological control is to be understood and manipulated.

•A monitoring or census system to ensure early detection of outbreaks should be in place, so that control measures can be planned, and applied at the most appropriate time.

•Economic damage and action thresholds should be established, so that control measures are not taken unless and until they are necessary, giving the nat- ural balance a chance to be re-established.

•Control measures must be selective, so that swift re-establishment of the natural balance is promoted.

Each of these aspects is discussed briefly below. For more detailed discussion in relation to oil palms, see Wood (1971, 1976c, 1979, 1987). Recent reviews are given by Teh (1996), Ho and Teh (1997) and Chung and Sharma (1999). Specific examples are also mentioned in the sections on individual pest species. Oil palm workers have not always adhered to IPM principles, though; numerous papers have been published describ- ing potential oil palm pests and testing insecticides for control, without any evaluation of the severity of damage or the extent of natural control. Many of the older recommendations took no account of the possi- bilities of IPM. In some instances these have not been updated, probably because it has become apparent that control by natural enemies is sufficient, and no action is necessary.

12.2.1.1 Pest ecology

As Wood (1976c) pointed out, the environment in an oil palm plantation, with a uniform expanse of the crop and a more or less constant environment, would appear to favour the build-up of pests. Many occasional pests are commonly present in a stand of oil palms, and the reason that outbreaks do not usually occur is that numbers are restricted by the action of natural enemies; these include both predators on the pests and species that parasitise the pests. Outbreaks may occur if the natural balance between a pest and its enemies is upset for any reason.

One of the major factors in pest outbreaks in oil palms in the past was the use of broad-spectrum (killing a wide range of species), persistent-residue, contact insecticides. These were often applied as a precaution- ary measure against minor pest infestations, but their

Table 12.11 Potential oil palm pests

Pest

Stage attacked

Damage

Occurrence

Ref.

Comments

 

 

 

 

 

 

Pests causing leaf damage

 

 

 

 

 

Red spider (Olygonychus sp.)

Nursery

Leaf turns brown, necrotic

Widespread

1

Often induced by pesticide application. Not

 

 

 

 

 

a problem with overhead sprinkler irrigation

Aphids

Nursery

Growth rate reduced

Malaysia

1, 2

Severity has not justified treatment

 

 

 

Congo

 

 

Mealy bugs, scale insects

Nursery, field

Leaf, fruit

 

2, 3

Often tended by ants; control of ants may

 

 

 

 

 

eliminate pest; serious damage rare

Grasshoppers

Nursery

Defoliation

Widespread

2

Several species; hand-removal effective

Grasshopper, Valanga nigicornis

Young palms

Defoliation

Malaysia

2, 4

Particularly in new plantings, after drought;

 

 

 

 

 

spraying may be necessary

Stink locust (Zonoceros variegatus)

Young palms

Defoliation

West Africa

2, 5

Damage followed slashing of overgrown covers

Leaf-cutting ants

Mature palms

Defoliation

Latin America

2

 

Alurnus humeralis

Mature palms

Defoliation

Ecuador

6

Coleoptera: Chrysomelidae: Hispinae

Homophylotis catori

Mature palms

Defoliation

Africa

5, 7

Caterpillar: Zygaenidae (syn. Chalconycles)

Leptonatada sjöstedti

Mature palms

Defoliation

Africa

5

Lepidoptera: Notodontidae

Hispoleptis spp. (Hispinae)

Mature palms

Leaf miner

Ecuador

2, 8, 9

Damage similar to Coelaenomenodera

 

 

 

 

 

lameensis; control by trunk injection (Ref. 9)

Promecotheca cumingi

Mature palms

Leaf miner

Malaysia

1, 8

Coconut pest, occasionally attacks oil palm

 

 

 

 

 

Coleoptera: Chrysomelidae: Hispinae

Norape argyrrhorea

Mature palms

Defoliation

Peru

18

Lepidoptera: Megalopygidae; controlled

 

 

 

 

 

by virus

Beetles

Mature palms

Bore into petioles

Malaysia, Africa

2

Xylotrupes gideon, Platygenia barbata

Retracus elaeis, orange spotting mite

Field

Orange spots

Colombia

10

Eriophidae; may cause 50% crop loss; wettable

 

 

 

 

 

sulfur gave effective control

Pests damaging trunk or roots

 

 

 

 

 

Eldana saccharina

Nursery

Larvae bore into spear/bud

Africa

5

Lepidoptera: Pyralidae

Dynastes (Augosoma) centaurus

Nursery, field

Similar to Oryctes

Africa

2, 5

Coleoptera: Dynastinae

Scapanes australis

Young field palms

Similar to Oryctes

SE Asia

11

Coleoptera: Dynastinae

Sufetula spp. (Lepidoptera: Pyralidae)

Mature palms

Destroy aerial roots

Widespread

5, 12

No evidence that loss of aerial roots is harmful

Monolepta apicicornis

Mature palms

Mines within root

West Africa

5, 13

Coleoptera: Chrysomelidae: Galerucinae

Termites (Coptotermes curvignathus)

Mature palms

Bore into trunk, palm falls

Malaysia,

1, 11, 14 Should be eliminated at time of planting; may

 

 

 

Indonesia

 

be a particular problem on peat (Ref. 15)

Cockchafer larvae

Nursery, field

Root damage

Malaysia

1, 16

Leucopholis, Apogonia, Adoretus

Cockchafers

Nursery, field

Leaf damage

Malaysia

1

 

Pests damaging fruit

 

 

 

 

 

Tiquadra spp.

Mature palms

Spear, bunch

Colombia

2

Damage similar to Tirathaba (12.2.8.1)

Prosoestus spp.

Mature palms

Stigma damage,

West Africa

2, 17

Effect small, undamaged fruits enlarge to fill

 

 

poor fruit set

 

 

space of damaged; treatment expensive

Elaeidophilos adustalis

Mature palms

Stigma damage,

West Africa

17

Effect small; treatment expensive

 

 

poor fruit set

 

 

 

Scale insects

Mature palms

Not serious

 

1

 

 

 

 

 

 

 

References: 1: Wood (1968a); 2: Hartley (1988); 3: Ponnamma (1999); 4: Han and Chew (1978); 5: Mariau et al. (1981); 6: Mariau (1976b); 7: Airede et al. (1999); 8: Mariau (1999a); 9: Le Verdier and Genty (1988); 10: Genty and Reyes (1977); 11: Mariau et al. (1991); 12: Genty and Mariau (1975); 13: Mariau and Djob Bikoi (1990); 14: Mariau et al. (1992); 15: Lim and Silek (2001); 16: Wood and Ng (1969); 17: Philippe (1993); 18: Zeddam et al. (2003).

426

The Oil Palm

 

 

 

 

Fourth generation

First generation

 

 

 

 

Second generation

Pretreatment

 

 

14

Fourth generation

First generation

 

 

Second generation

Pretreatment

 

14

 

 

 

 

 

 

 

12

 

 

 

12

 

 

 

 

 

10

 

 

 

leaf10

8

 

 

per leaf

per

 

 

 

8

 

 

 

Larvae

6

 

 

Larvae

6

 

 

 

 

4

 

 

 

 

 

 

 

 

 

 

 

4

 

 

 

 

 

 

 

 

2

 

 

 

 

 

 

 

 

2

 

 

 

 

 

 

 

 

0

 

 

 

 

 

 

 

 

315

225

135

45

Sprayed

45

135

225

315

0

Distance from sprayed area (m)

315 225 135 45 Sprayed 45 135 225 315

Fig. 12.3 Effects Distanceof dieldrinfromspraysprayedon areapopulation(m) of Metisa plana. The population increased after spraying, centred on the sprayed area. (From Wood, 1971.)

use could exacerbate the problem, by killing the natural enemies of pests, so that any biological control ceased to operate. The pest which became a problem after spray- ing was not always the same as that which had provoked the initial precautionary application. The possible effect of spraying with a broad-spectrum chemical is clearly illustrated in Fig. 12.3. When a 0.8 ha plot of palms was sprayed regularly with a low dosage of dieldrin, the population of the bagworm, Metisa plana, built up con- siderably over the next four generations, spreading out from the sprayed area. Wood (1968a) noted that it is dif- ficult to prove that insecticides have caused outbreaks, but the coincidence has been sufficiently frequent as to leave little doubt.

One reason why spraying with broad-spectrum chem- icals can cause a pest outbreak is that it is likely that a few of the pests will survive the spraying, but almost none of their enemies, simply because the pest is usually present in larger numbers than its enemies. In that case, the pest can then build up uncontrolled. With contact insecticides, patchy or uneven application is likely to make matters worse, because the natural enemies are usually mobile and exploratory in their behaviour, so will move into sprayed patches, while the more static pest remains untouched elsewhere. Wood (1987) noted that a high kill of the pest with moderate survival of enemies was probably better than a moderate kill of the pest with complete survival of enemies. An additional factor is that only one particular stage in the life cycle of

the pest may survive; if so, then subsequent generations will be highly synchronised (e.g. Mariau, 1976a). Under normal conditions in the tropics, all stages tend to be present, and there has thus been no pressure for the natural enemies to evolve co-ordinated life cycles. If pest generations do become synchronised, natural enemies may be heavily outnumbered, so that control of the pest breaks down.

Pesticides are not the only cause of outbreaks, which sometimes occur in areas where pesticides have not been applied. Syed and Shah (1977) quoted circum- stantial evidence to suggest that complete eradication of weeds by herbicide spraying had contributed to pest outbreaks in Sabah. They argued that weed species were important food plants for the adults of parasitic wasps; weed eradication thus reduced the level of para- sitism and allowed pest numbers to build up. Prior (1988) demonstrated the importance of two weed species in supporting a wasp parasitic on grasshoppers in PNG (Section 12.2.5.4). Delvare and Genty (1992) recommended protecting and spreading certain weed species in Latin America, to support beneficial insects. In Malaysia, Ho (1998) showed that numbers of the bagworm, Metisa plana, were suppressed for about 200 m on each side of a planted strip of Euphorbia heterophylla, a species shown to be attractive to a wide variety of different insects.

Wood (1968a) suggested that dust from dry dirt roads in plantations might be a disturbing factor. Dust can kill insects by abrasion of the cuticle, and may affect the active natural enemy species more than rela- tively inactive pests such as caterpillars. Syed and Shah (1977) considered that any effect of this would only extend a few palms away from the road, but that pockets with high pest numbers might develop in consequence, which could then lead to larger outbreaks when other conditions were favourable. Siburat and Mojiun (1998) observed outbreaks of leaf-eating caterpillars after floods, which might have eliminated natural enemies whose adults lived on ground vegetation. They suggested that drought could have a similar effect.

Knowledge of pest life cycles can be useful, particu- larly when synchronised generations occur. Life cycles for many Malaysian pests were given by Wood (1968a). Siburat and Mojiun (1998) gave a life-cycle table for Setora nitens, showing the control measures appropriate for each stage. Studies of natural enemies can also be useful, although the enemies important under normal conditions may not be the most effective in containing outbreaks (Wood, 1979). Mariau et al. (1991) illustrated a number of the more important predators and para- sites in South-east Asia.

Diseases and Pests of the Oil Palm

427

12.2.1.2 Economic damage thresholds

Response should be related to pest numbers, rather than following a predetermined programme. Regular prophylactic pesticide applications can appear to give control, but by removing natural enemies, may ensure that the problem persists. Defining a critical level of a pest provides the necessary link between ecology and economics (Wood, 1979). A warning level may be use- ful, as an alert to potential danger. It is important also to understand whether the pest population is increas- ing, stable or decreasing.

As an example, studies of the damage caused by Oryctes in young oil palms have shown that, during the first year in the field, quite extensive defoliation has lit- tle or no effect on subsequent yield (Wood et al., 1973; Liau and Ahmad Alwi, 1995). This appears to be because growth at that stage is not limited by photo- synthetic activity, so after partial defoliation, the remaining leaves can photosynthesise more rapidly to meet requirements for vegetative growth (see Section 4.2.2.1 for further discussion of this). Some degree of control of Oryctes is needed, because the beetle may kill young palms, not just defoliate them. In addition, if damage persists into the second year, it starts to affect yield (Liau and Ahmad Alwi, 1995; Chung et al., 1999). The important point, though, is that apparently severe damage in the first year may have little economic effect, and the control strategy should take account of this.

12.2.1.3 Census systems

If responses are to be based on pest numbers, then a regular monitoring system must be in place. Pest num- bers may be counted directly, or an assessment of damage may be made. Monitoring systems for leaf- eating caterpillars were discussed by Wood (1976c). Most systems involve at least two stages: a superficial inspection for signs of pest incidence, the ‘detection’ stage, and more detailed assessment where such signs are found, the ‘enumeration’ stage. Chung et al. (1995) described a system for bagworms, based on unpub- lished work by Wood, with three stages.

1.The alert stage involves looking for the small holes in leaves, which are signs of feeding damage. This inspection can be undertaken by harvesters, and is frequent (fortnightly) but superficial.

2.When feeding damage is noticed, the census stage is activated. An upper frond from 1% of palms is cut down, and the number of individual bagworms and nettle caterpillars, both alive and dead, is counted by a specially trained team. The census is repeated

in affected areas every 1 or 2 weeks, different palms being used each time to avoid excessive defoliation.

3.The results of the census determine whether the action stage is needed. Control measures are imple- mented if the census in affected and adjacent blocks shows that all of the following conditions are met.

•Infestation is spreading.

•Natural control appears to have failed.

•The pests are at the small larval stage, with cocoons absent or very few.

•Numbers exceed 5–10/frond for the smaller species, or 1–5 for the larger, indicating that there is a high risk of crop loss.

Mariau (1994) gave a more general review of census systems in various parts of the world, together with a list of advice notes on pests published in Oléagineux between 1967 and 1994. The recommended frequency of the alert stage depends on the expected pest species. The choice of leaf for detailed recording will depend in part on the particular pest, as some damage young leaves and some older leaves (Wood, 1976c). The action level for a pest may depend on the weather; Mariau (1999b) noted that the leaf miner Coelaenomenodera lameensis multiplies more slowly during drought, so the action level can be higher.

12.2.1.4 Control measures

Various management practices are important compon- ents of IPM, in ensuring that outbreaks do not occur. The probable importance of the weed flora as food plants for natural enemies has already been mentioned. Ground cover also plays a role in limiting Oryctes dam- age (Section 12.2.4.1). Study of the life cycle of a pest may reveal an alternate host plant, the elimination of which from the plantation will help to control the pest.

If pesticide application is needed, it should be select- ive. Bio-insecticides (preparations of parasitic fungi or insect viruses, or Bacillus thuringiensis) can be effective (e.g. Desmier de Chenon et al., 1988; Sipayung et al., 1990; Ramle et al., 1995). The damaging effects of some of the older broad-spectrum, persistent-residue chemicals have already been mentioned, but some mod- ern chemicals are also broad spectrum and can cause problems if wrongly used. The synthetic pyrethroids, for example, break down rapidly and have little residual effect in the tropics, but they are broad spectrum and repeated use can damage natural enemy populations.

Selectivity can be achieved not only by choice of chemical, but also by the timing and method of appli- cation. Knowledge of the pest life cycle may allow application to be timed to a stage when the maximum

428

The Oil Palm

kill can be achieved, while sparing natural enemies. For example, with Coelaenomenodera lameensis, spraying may be most effective against adults; larvae in galleries within the leaf lamina are protected from contact pesti- cides (Mariau, 1999b). One of the best methods of insecticide application to oil palms is by trunk injection (Wood et al., 1974) or root absorption (Ginting and Desmier de Chenon, 1987). In this instance, the appli- cation method provides selectivity. A systemic pesticide moves into the leaves, and only species that eat the leaves will be affected. The risk of affecting non-target species is usually minimal, although Prior (1988) noted effects on crows and domestic chickens after they had eaten grasshoppers that had been treated by trunk injec- tion of monocrotophos. Sarjit (1986) showed that trunk injection could be done effectively and economically on a large scale. The equipment is cheap and may have other uses. This is important, as where pest outbreaks are rare, equipment may remain unused for long periods.

Insect pheromones, volatile chemicals produced as signals to other insects, are increasingly being used in pest-control strategies. For example, female bagworms are wingless and immobile, and attract males by releasing a pheromone. Baiting traps with the pheromone to col- lect males could, clearly, disrupt mating and contribute to control of bagworms (Rhainds, 2000). Pheromone- baited traps may also be used for monitoring populations.

affected (Buyckx, 1952). The eggs are laid by the moth at the base of the spear leaves and even one larva hatch- ing can do considerable harm. Usually, two or three are found on young palms or up to a dozen on older ones and, typically, they penetrate the rachis and leaflets of the growing, unopened spears, forming galleries through them. The attack may proceed downwards towards the growing point and the rachis may be so damaged that later, in a strong wind, several young leaves may snap near the base. When unbroken spears open, the holes left by the caterpillar are seen to be symmetrically placed on either side of the rachis. The caterpillar does not kill the palm, but may be followed by weevil larvae, e.g. those of Temnoschoita, or a bacterial rot that may prove lethal (Buyckx, 1952).

The caterpillars reach a length of 3–4 cm before pupating in a cocoon of fibrous debris. The colour of the caterpillar changes from dark red to yellowish as it devel- ops. The olive to brown moths are not long in emerging from the pupae and the whole life cycle takes 35–45 days; attacks can therefore be made at frequent intervals.

Control: Light attacks can be dealt with by removal of infected leaves and collection of the caterpillars and pupae. In the Ivory Coast spraying has been carried out at intervals of 2–3 weeks in nurseries and for the first 2 years in the field, but older palms are less vulnerable to attack and are not usually treated (Mariau and Morin, 1971).

12.2.2 Nursery pests

Several potential nursery pests are listed in Table 12.11. Red spider mite may have severe effects, but is rarely a problem in nurseries irrigated by overhead sprinklers. Boron applications have been found to reduce spider mite injury (Rajaratnam and Law, 1975).

12.2.3 Leaf pests of immature palms

Most of the leaf pests of mature palms may also attack young palms. The pests listed in this section appear to attack mature palms only rarely, however.

12.2.3.1 The African spear borer

Distribution: This moth, Pimelephila ghesquierei

(Pyralidae, Pyraustinae), first described from Congo, is found in all African territories. It is not a common pest, but on occasions the damage done has been severe. It has perhaps been more troublesome in Central than in West Africa.

Life cycle and damage: Damage is most common between the second and fifth years in the field, but both nursery seedlings and older palms are sometimes

12.2.4 Stem damage to young palms

12.2.4.1 Oryctes species (Dynastinae): rhinoceros beetles

Distribution: Species of Oryctes are found throughout the palm-growing areas of Africa, Asia and the Pacific. Damage is worst in young palms, and comparatively rare in palms older than 3 or 4 years, although Dhileepan (1988) recorded damage up to 15 years after planting in India, and isolated tall palms are often attacked.

The following species are the most important oil palm pests:

O. rhinoceros

The common rhinoceros beetle

 

of the Far East, which has

 

spread to the Pacific islands

 

(Plate 12.5)

O. gnu

Asia, less common

(O. trituberculatus)

 

O. boas

Probably the most common

 

species in Africa

O. monoceros

Africa

O. owariensis

Africa

Diseases and Pests of the Oil Palm

429

Plate 12.5 Oryctes rhino- ceros in Malaysia. (A) Adults, male (right), female (left).

(B) Larval instars, 1st, 2nd, 3rd (early, late, prepupal).

(C) Head capsules of larval instars; (left to right) 3rd, 2nd, 1st.

The Asian species are primarily pests of the coconut palm, but they attack other palms, both cultivated and wild. The African species attack the coconut and Borassus palms but, owing to its ubiquity, the largest population is to be found on the oil palm.

Description, life cycle and damage: The male adult has the characteristic ‘rhinoceros horn’; in the female the horn is smaller or, in the African species, is reduced to a triangular protuberance. The beetle is black and measures 4–6 cm long and 2–3 cm broad according to species, O. trituberculatus being larger than O. rhinoceros, and the African species O. owariensis being larger and

O. boas being smaller than O. monoceros. The horn of O. boas is particularly long and curved.

The eggs are white, 3–4 mm in diameter and easily observed on breeding sites. About 20 eggs are usually laid, but higher numbers have been recorded; they hatch after 11–13 days. The young larva is white at first, but its head soon becomes brown and its body blue–grey, then yellowish or greenish-white; it reaches a length of 4–10 cm. The duration of the larval stage varies considerably, ranging from around 100 to 200 days. Similarly, the adult stage may last for a few months or extend to over half a year. Before pupation there is a

430

The Oil Palm

short prepupal stage of a week; the adults emerge after a further 3 weeks. The eggs are laid on rotting vegetable matter. On an oil palm estate decaying palm trunks and bunch refuse are common breeding grounds. Empty bunches stacked prior to use as mulch may be a prob- lem; the length of the larval stage is such that spreading should usually be done before pupation takes place, but the life cycle may sometimes be completed.

The oil palm is damaged by the adult beetle, which burrows into the cluster of developing spears in the crown and bores its way through the petioles into the softer tissues of the younger, unopened leaves. The effect can be seen when these leaves develop and open, but the regularity of wedge-shaped cuts so characteris- tic with the coconut palm is not always so clearly seen in the oil palm. Where the rachis has been penetrated, leaves may later snap off. Previous attacks may be detected by the presence of holes in the petioles of older leaves. Attack is most damaging in young palms, since the growing point is occasionally reached, or bud rot may develop and kill the palm. The holes may also give Rhynchophorus access to the palm (Section 12.2.6.1), with Rhynchophorus damage, in turn, providing condi- tions suitable for Oryctes larvae (Zulnerlin and Fatah Ibrahim, 1999).

Apart from the occasional young palm that is actu- ally killed, the effect of Oryctes damage on yield may be very small. Wood et al. (1973) compared two 2.4 ha plots, one with over 80% severe or medium damage by Oryctes, and one with only 6%, and found that the former yielded 2% less over the first 18 months of production. In this trial, damage probably ceased at least a year before harvesting started. Later damage can have more effect. Liau and Ahmad Alwi (1995) found that artificial defoliation of 2-year-old palms did affect yield: 50% defoliation on a single occasion reduced subsequent yield by 12%, and repeated (‘chronic’) defoliation reduced it by 24%. Chung et al. (1999) found that palms damaged by Oryctes 21 months after planting yielded 80% less than undamaged palms in the first 12 months of production (although this com- parison was based on groups of only 20 palms). With more than 70% of palms damaged, they estimated a loss of 2.1 t FFB/ha in the first year. Liau and Ahmad Alwi (1993) observed less damage in the second year in the field, and harvested 5 t more fruit over the first 18 months of production from palms in a clean clearing than from plots with windrowed trunks.

Control: Older methods of control involved destruc- tion of breeding sites, and hand-collection of adult bee- tles. Hartley (1988) advocated that all rotting vegetable matter should be dispersed and rotting palm trunks

disposed of. Where the palm trunks were not burnt, regular inspection to break up the rotting material and collect larvae was recommended (Barlow and Chew, 1971). This method was labour intensive, but Zulnerlin and Fatah Ibrahim (1999) found that in Indonesia hand-picking was cheaper than insecticide application. In some countries, the larvae are hand-picked and cooked and eaten as a delicacy. Complete pulverising of trunks to sawdust-sized particles, thus largely eliminat- ing breeding sites, has been claimed to reduce the pop- ulation to less than 3% of that with the usual chipping, which cuts the trunk into fragments weighing 1.5–8 kg (Ooi et al., 2001).

It was noticed in Malaysia that palms along or near roadsides might be heavily attacked while those within the field escaped injury, and Wood (1968b) showed that interrow vegetation may from a barrier to beetle move- ment and, in young areas, may blur the palm silhouette which is believed to attract the beetle. When areas of young palms either kept bare or sown with a legume cover crop were compared, Oryctes breeding and damage were considerably higher on the bare areas (Wood, 1968b). In addition to its many other advantages, the planting of leguminous cover is undoubtedly an effective way of suppressing Oryctes attack. It has been confirmed that this approach is also effective against O. monoceros in Africa (Boyé and Aubry, 1973). Wood (1976a) noted that ground cover does not totally elimi- nate attack, but reduces it to a very low frequency, such that effects on yield are unlikely to be important. He suggested that Oryctes attack is rare on older palms, because the closed canopy itself forms a ‘vegetative barrier’; as noted above, isolated older palms are often attacked.

Ethyl chrysanthemumate was found to be a strong attractant to Oryctes, and Turner (1973) suggested bait- ing traps with this compound, but Wood (1976a) thought that the density of traps required (25/ha) was too great for the method to be cost-effective. Hallett et al. (1995) found that the aggregation pheromone, ethyl-4-methyloctanoate, was ten times more attractive to Oryctes than ethyl chrysanthemumate. This allows a much lower trap density, and Chung (1997) showed that one trap per 2 ha gave good control of damage, provided that the Oryctes population was not too large. Costs were lower than for insecticide application. Where risk of attack is thought to be high, a systemic insecticide (carbofuran) may be applied. Desmier de Chenon et al. (1998) advocated trapping in the old stand for 6 months before replanting, to reduce the population. N. Kamarudin et al. (1999) suggested using pheromone trapping to monitor beetle populations and

Diseases and Pests of the Oil Palm

431

identify ‘hot spots’ where chemical control would be worthwhile.

Much attention has been paid to the introduction and spread of insect parasites, fungi or viruses. A virus, Rhabdionvirus oryctes, first identified in Malaysia (Huger, 1966), has been introduced over most of the South Pacific where O. rhinoceros is a coconut pest. Infected beetles stop feeding, and fly and mate less frequently (Zelazny, 1977), and beetle populations have been reduced follow- ing introduction of the virus (Young, 1986). In Malaysia, there is a regular, although small, larval mortality from the virus (Barlow and Chew, 1971), and it is likely that the virus and its host have reached an equilibrium. Barlow and Chew (1971) also identified a fungus, Metarhizium anisopliae, which infects the larvae; Tey and Ho (1995) applied cultures of this to Oryctes breeding sites and achieved high infection rates, coincident with reduced numbers of larvae. Ho (1996) recommended an inte- grated approach to Oryctes control, involving pheromone traps, release of trapped beetles inoculated with Metarhizium, inoculation of breeding sites with the fun- gus, and the use of synthetic pyrethroids when damage reached unacceptable levels. He quoted costs equivalent to about 3 t FFB/ha in the first year of production. Given the rather small effects of Oryctes damage on yield, noted above, it must be doubtful whether such measures are justified. It should also be noted that Hochberg and Waage (1991) used a model to show that, if Metarhizium were to be applied to a population already regulated to low levels by a virus, the ensuing mortality may result in the virus being eliminated from the population, so that a later resurgence of Oryctes may occur.

There is a view that the adoption of zero-burn replanting methods has increased problems with Oryctes, but we should not forget that, until about 30 years ago, all replants were zero-burn. The belief that the old stand must be completely uprooted to prevent Ganoderma attack (which may have little foundation; see Section 12.1.6.3) led to burning or windrowing of the old trunks. Where windrowing was done, if the old trunks were not adequately covered by vegetation, Oryctes could invade. Wood (1999) noted that there have been Oryctes outbreaks in recent years, which have led some to question the ‘vegetative barrier’ effect men- tioned above, but he considered that in such cases there was always some evidence that development of vegeta- tion cover had been restricted, for example by felling too late after poisoning, by stacking the palm trunks well above the vegetation, or by flooding.

Kamarudin and Basri Wahid (1997) surveyed 640 estates in Malaysia, with a total of 280,000 ha of imma- ture palms. Oryctes infestations were reported from

16% of the area, with 13% of outbreaks described, sub- jectively, as ‘serious’ or ‘very serious’. However, only 3% of outbreaks were in palms older than 18 months, and we have seen that before that age damage has little effect on subsequent yield.

Samsudin et al. (1993) found much higher Oryctes numbers in an underplanting than in an area where the old stand had been felled and trunks chipped before planting. Incidence was particularly high if the under- planted old stand was poisoned and left standing, as expected from the absence of vegetation cover. Similar observations have been used as an argument against underplanting (Section 8.3.4.3), but provided that the old stand is felled and quickly covered by ground vege- tation, there should not be a problem (Hakim et al., 1998). If Oryctes does built up in an underplanting, though, there could be significant effects on yield. If the last of the old stand is cleared 2 years after under- planting, there might be damage to palms aged 2 years or more, and damage at that age has been shown to affect yields (see above). Jacquemard et al. (2002a) found large differences between oil palm families in amount of damage (though with no indication of statis- tical significance), and considered that some genotypes were more attractive to Oryctes than others. While this might not be important for seed production, they sug- gested that it should be considered in clone selection.

Conclusion: If a good legume cover is established early in an oil palm replant, this effectively suppresses Oryctes, and the amount of damage to young palms should be small. As it has been clearly established that quite severe defoliation during the first year in the field has little effect on subsequent yield, in most situations it is unlikely that Oryctes control measures will be needed. Only if severe damage continues beyond the first 18 months in the field would the possible loss in yield justify treatment.

12.2.4.2 Strategus aloeus (Dynastinae)

This beetle, which somewhat resembles Oryctes, is dis- tributed throughout tropical America, where it has been troublesome in several oil palm plantations (Mariau, 1976b).

Incidence and damage: The adults attack young palms in the field or nursery by digging a hole in the ground near the palm, from which they bore their way into the plant just above the roots. Often, in a young palm, the growing point is reached and the plant killed. Eggs may be laid in the palm, which is then consumed by the developing larvae, or in rotting stumps, trunks and vegetation.

432

The Oil Palm

Control: In view of the lethal attack on young palms by the adult, control measures are required in areas where the beetle is common. As eggs may be laid in rot- ting stumps and trunks, the measures to be taken against the larval stage are the same as for Oryctes species: destruction of breeding sites and collection of larvae. During wet weather fortnightly inspections have been recommended; in dry weather Strategus attack is rare.

It does not appear to be recorded whether the ‘vegeta- tive barrier’ effect which controls Oryctes is effective against Strategus.

12.2.4.3 Temnoschoita species (Curculionidae)

These weevils are found throughout Africa, but appear to be more common in Congo than in West Africa. The most common species is T. quadripustulata (T. quadrimac- ulata); T. delumbata is less common. Young palms may be killed, but the pest appears to cause no significant damage to mature palms (Mariau et al., 1981) (Plate 12.6).

Life cycle and damage: Asante and Kumar (1986) described the life cycle of T. quadripustulata in Ghana. The adults are 8–10 mm long and dark brown, with the thorax spotted with indentations. The light brown wing cases have four reddish blotches and do not fully cover the abdomen. The females lay their eggs on cuts and wounds on the leaf petioles. On young palms, both those recently transplanted and palms in early bearing, and on nursery plants the young larvae tunnel their way through both dead and living tissue towards the heart of the palm, and pupate in the tunnels formed. The damage is sometimes severe, and young palms may be

Plate 12.6 Temnoschoita damage to leaves in West Cameroon. Note typical ‘windows’.

killed through penetration to the growing point. In bearing palms the adults are attracted to the inflores- cences, where eggs are also laid.

Control: In areas where the weevil has been noted, care should be taken to avoid wounding the palms by excessive leaf pruning, particularly just before trans- planting (Buyckx, 1952). This injunction may conflict with control measures against Cercospora leaf spot (Section 12.1.2.1), and it may be useful to treat pruning cuts with tar. With bearing palms the collection and destruction of rotted bunches and scattered fruit are also recommended, as these may contain eggs, larvae and pupae. When harvesting begins it may be advanta- geous to undertake a general cleaning of the crown fol- lowed by dusting with an insecticide at intervals of 3 weeks, the dust being applied in the crown from the centre to the base, not on the leaves. Traps for the adults have been constructed from recently cut and split petioles or banana trunks (Buyckx, 1952). Banana plants are an attractive host and should not be grown near nurseries or young plantations where infection with Temnoschoita is feared.

12.2.5 Leaf pests of mature palms

Numerous species eat leaf tissue of mature palms and have the potential to cause significant defoliation. The effects of this on yield were investigated by Wood et al. (1973), who used manual defoliation to simulate the effect of attack by a leaf-eating pest on 8-year-old palms. Figure 12.4 shows that, in the first year after defoliation, 50%

 

50

 

 

 

Upper leaves

first year

40

All leaves

 

Lower leaves

30

 

(%) in

 

 

 

Yield loss

20

 

10

 

 

 

 

0

 

50

25

12.5

6.25

 

Percent of leaflets removed

 

Fig. 12.4 Effects of artifical defoliation on subsequent yield. Different degrees of defoliation were applied by stripping off leaflets, to upper leaves, lower leaves or all leaves. (From Wood et al., 1973.)

Diseases and Pests of the Oil Palm

433

damage caused a crop loss of over 40%, if restricted to the upper half of the canopy (i.e. all leaflets in the upper half destroyed). This corresponded well with obser- vations in a severely attacked area. Damage to the lower leaves had comparatively little effect, because those leaves are shaded by younger leaves at the top of the canopy, and contribute relatively little to total canopy photosynthesis. Less severe damage caused smaller losses.

The most severe treatment caused a further 17% crop loss in the second year. Study of monthly yields showed that there was a large shortfall about 10 months after defoliation, attributable to inflorescence abortion, and a further shortfall between 22 and 26 months, due to an effect on sex ratio (see Chapter 4 for discussion of these yield components). A mature oil palm canopy consists of about 40 leaves, and as leaf production rate is around 24 per year, the palm will take over a year to recover from damage to the younger, upper leaves.

This study allows economic damage levels to be esti- mated for any pest that causes defoliation. Wood (1977) noted that the cost of treatment by aerial spraying or trunk injection would be covered by a yield gain of no more than 3%. Figure 12.4 suggests that even quite mild defoliation could cause a loss of that order. The question, then, is not whether the cost of spraying will be recovered, but whether the outbreak will come under natural control anyway. If surveys indicate that numbers are increasing, and if a proven integrated control meas- ure, such as trunk injection or an insect pathogen, is available, it will be advisable to take action at a fairly early stage.

12.2.5.1 Leaf-eating caterpillars

A wide variety of different caterpillars feeds on oil palm leaves. Nettle and slug caterpillars and bagworms are the most frequent pests. Mariau et al. (1991) presented good colour photographs of most of the important species, and of their natural enemies, but little information on control. Syed and Saleh (1993) described IPM systems for these pests in Indonesia.

Nettle and slug caterpillars: Members of this group, the Limacodidae, have been recorded as pests in all oil palm-growing areas. Wood (1987) listed the species that have been recorded as causing significant damage to oil palms (Table 12.12) and other crops. Some species, such as Darna trima, are common, yet severe damage may be unusual. Outbreaks of several species have been attributed to prior use of contact insecticides.

Life cycles and damage: Severe infestations may develop rapidly, as the life cycles are only a few weeks long and reproduction rates are high. The eggs are

deposited on the leaflets; the caterpillars usually feed on the underside, often stripping the surface when young, but when larger they may eat away the whole lamina, leaving only the midrib (Plate XVIA). Pupation may be on the ground, in cracks in the soil, or attached to the leaf.

Mariau et al. (1991) gave critical levels, in terms of number of larvae per frond, for most species in South- east Asia, together with recommendations on insecti- cides. Different species attack different parts of the crown: D. trima is first found on leaves 9–17, Setothosea asigna and Setora nitens (Plate XVIB) on leaves 9–25, and Ploneta diducta and P. bradleyi on older leaves (Mariau et al., 1991). In severe outbreaks the entire crown may eventually be defoliated. The effect on yield depends on the extent of defoliation, and on which part of the crown is damaged. As noted above, loss of young leaves is more damaging than loss of older leaves.

Control: Numerous parasites and predators of the Limacodidae have been recorded. Species attacking S. nitens (Plate XVID) included five species of wasp, four parasitic flies and a bug (Wood, 1966, 1968a). Setothosea and related species are attacked by a virus, a fungus (Cordyceps sp.), and several predatory or para- sitic insect species (Tiong, 1979).

Wood et al. (1977) described a series of experiments on the control of nettle caterpillars in Malaysia. They tested a range of chemicals and Bacillus thuringiensis insecticides, and considered the latter to be very prom- ising. The trials confirmed the need for chemical inter- vention on occasion, choosing insecticides on grounds of good kill, selectivity, low cost and low toxicity to humans. The utilisation of diseased pests by spraying suspensions of crushed bodies was in several cases very successful. Tiong (1982) described control of Setothosea asigna by an integrated programme similar to that advocated by Wood et al. (1977), in which chemical inter- vention was confined to quelling high-density infestation and combined with the encouragement of the natural fungal and insect enemies. In Sarawak, good progress has been made with the control of Darna trima. A virus inoculum was prepared from diseased late-instar larvae, and from healthy larvae confined with them. The larvae were naturally infected by the virus and the inoculum was prepared by simple maceration, straining and dilu- tion with water. This was sprayed with mist blowers, repeated until the larval census showed that resurgence was not taking place. There was a high mortality within 8 days in comparison with unsprayed areas, and resur- gence, such as was common after 5 weeks with chem- ical insecticide spraying, did not take place (Tiong and Munroe, 1977).

434

 

 

 

 

The Oil Palm

 

 

 

 

 

Table 12.12 Nettle and slug caterpillars reported to have reached outbreak levels on oil palms

 

 

 

 

 

 

 

 

 

 

Species

Synonym

Location

Frequencya

Ref.

 

Susica malayana

S. pallida

W Malaysia

Occasional

1

 

 

Setothosea asigna

Thosea asigna

W Malaysia, Sabah

Often

2

 

 

Setora nitens

 

W Malaysia, Sabah, Sumatra

Frequent

2, 3

 

 

Birthosea bisura

Thosea bisura

W Malaysia

Occasional

2

 

 

Ploneta diducta

Darna diducta

W Malaysia

Occasional

2, 3

 

 

Darna trima

 

W Malaysia

Often

2, 3

 

 

 

 

Sabah

Often

4

 

 

Darna mindanensis

Darna sp. nr. trima

Philippines

Often

2

 

 

Darna furva

 

South Thailand

Often

5

 

 

Darna catenatus

 

Sulawesi

Often

6

 

 

Thosea vetusta

 

Borneo

Occasional

3

 

 

Parasa pallida

Latoia pallida

West Africa

Often

7

 

 

Parasa viridissima

Latoia viridissima

West Africa

Often

7

 

 

Episibine intensa

 

Guyana, Colombia

Frequent

8

 

 

Episibine sibinides

 

Peru

Often

8

 

 

Euclea diversa

 

Central & S America

Often

8

 

 

Euclea cuprostriga

 

South America

Often

8

 

 

Euprostema elaeasa

Darna metaleuca

Central & S America

Frequent

8, 9

 

 

Natada pucara

 

Central & S America

Occasional

8

 

 

Natada subpectinata

 

South America

Occasional

8

 

 

Sibine fusca

 

South America

Occasional

8

 

 

Sibine nesea

 

South America

Occasional

8

 

a Occasional: one or two recorded outbreaks only; frequent: extensive, heavy and recurrent outbreaks in more than one location, for some period of time; often: intermediate between occasional and frequent.

Based on Wood (1987).

References: 1: Wood (1968a); 2: Wood et al. (1977); 3: Ho and Sidhu (1986); 4: Wood and Nesbit (1969); 5: Wood (1987); 6: Mariau et al. (1991); 7: Mariau et al. (1981); 8: Genty et al. (1978); 9: Genty (1976).

The biology of Euprostema elaeasa, which is among the more serious of the numerous South American cater- pillar pests, was studied by Genty (1976), who found sev- eral important parasites, including a wasp, Casinaria sp. Genty recommended that the parasite population be carefully examined before control measures are decided upon. Sibine fusca, another American species, is attacked by several bugs, and Genty (1981) recorded a wasp,

Apanteles sp., and two flies, Palpexorista coccyx and

Systropus nitidus, which not only parasitise the larvae but also transmit a viral disease (Meynadier et al., 1977). This virus can be artificially spread: 20 g of infected larvae macerated in 220 ml of water and applied at 50 ml/ha was shown to spread the disease over the whole population within 18 days. Mexzón et al. (1996) controlled S. megasomoides in Costa Rica by spraying with B. thuringiensis and deltamethrin.

Species of Parasa have defoliated oil palms in West Africa. Specimens from Cameroon, Nigeria, Liberia and Uganda have been identified as P. viridissima, while P. pallida has been a pest of oil palms in the Ivory Coast (Mariau and Julia, 1973; Mariau et al., 1981). Normally,

fungi and natural predators and parasites keep the popu- lations in check by attacking the larvae and pupae. In the Ivory Coast, spraying 3 weeks after the appearance of the first caterpillars has been recommended (Mariau and Julia, 1973). Fediere et al. (1990) described the use of a virus for control of P. viridissima.

Monitoring: A pheromone produced by the female of D. trima has been identified by Sasaerila et al. (2000), who suggested that it could be used to trap males and hence to monitor the population. Desmier de Chenon et al. (1996) recommended using pheromone traps for monitoring populations of S. asigna. They indicated that this was cheaper than conventional census systems, and might give an earlier warning of pest build-up.

12.2.5.2 Bagworms

Several members of the Psychidae have been pests of the oil palm in Asia since the start of the plantation industry, but the prevalent species appear to have changed. In the period between World Wars I and II Mahasena corbetti was extensively studied, but since

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435

1945 Pteroma pendula (formerly Cremastopsyche pen- dula) and Metisa plana have been the common species in Malaysia. In Indonesia M. corbetti is the principal species, and it can be a serious pest throughout South- east Asia. As with nettle caterpillars, effects on yield will depend on the extent of defoliation.

Life cycle and damage: The larvae of bagworms are encased in bags constructed of pieces of leaf bound with silk. Metisa plana and P. pendula feed on the upper surface of the leaf, the scraped portion first becoming dried out and then forming a hole. Further damage is done by the removal of pieces of leaf to make the case. Badly damaged leaves soon dry up and this gives the lower and middle part of the crown a characteristic grey appearance, the only green leaves being the youngest. Mahasena corbetti feeds on the undersurface of the leaf. Surviving caterpillars pupate in their cases on the underside of the leaves.

The size and form of the bags and the manner in which they are attached to the leaf help to distinguish the species. Metisa plana has a short, hooked attachment and the bag is about 13 mm long (Plate 12.7). The case of P. pendula is about 6 mm long, is rather rough and hangs on the end of a long vertical thread. Mahasena corbetti is much larger and the case is more ragged (Plate XVIE); Syed et al. (1973) studied the life cycle of this species. The male moths of all species fly from their cases, but the females are wingless and remain in the case. They attract the males with a pheromone and mate while still in the case, each then laying 100–300 eggs. On hatching, the caterpillars acquire their own cases and feed in groups. The life cycles take between 2.5 and 4.5 months (Syed, 1978).

Kamarudin et al. (1996) described 18 species of para- sitoid associated with M. plana and M. corbetti in Malaysia. The caterpillars die in large numbers from parasitic and predatory attacks and other causes, but explosions of population may occur locally from time to time when the natural balance is disturbed for one reason or another. The probable role of contact insecti- cides, sprayed both against other minor pests and against the bagworms, in these population explosions has already been mentioned.

Control: Owing to the risks associated with broad- spectrum contact insecticides, hand-picking was long recommended for small attacks, and stomach poisons such as lead arsenate or trichlorfon (which has short- lived residues) for larger outbreaks. Aerial spraying over wide areas was successful with trichlorfon (Wood, 1968a). Later, the systemic insecticide monocrotophos, applied by trunk injection, was found to be effective (Wood et al., 1974). The injection was done by pouring

Plate 12.7 Bagworms in Malaysia: (left) Metisa plana; (right)

Pteroma pendula. (B.J. Wood.)

the chemical into holes made with modified chainsaw drills. Tractor-mounted generators with electric drills and special equipment for immediate injection follow- ing drilling may now be used (Sarjit, 1986); one team with a tractor and two drills can treat 15–20 ha in a day.

Syed and Saleh (1993) described a census system for M. corbetti. Syed and Saleh (1998) achieved effective control of this pest by spraying part of an infested area with a granulosis virus. Basri Wahid et al. (1996) tested preparations of B. thuringiensis against M. plana and M. corbetti. The best preparation was as effective as trichlorfon, but some were much less effective. Rhainds (2000) suggested that control should be possible using pheromones to trap males and disrupt mating; this has been done successfully with another bagworm species,

Thyridopteryx ephemeraeformis (Klun et al., 1986). Chung and Sim (1993) discussed a situation where

bagworms had become a constant problem in a particu- lar area, and were apparently not easily controlled. They showed that standard procedures (census twice per month, trunk injection where the threshold of ten lar- vae per leaf was exceeded), provided they were correctly followed, could reduce an apparently chronic infestation within a few months to a level where natural enemies took over control.

Ho (1998) demonstrated the value of Euphorbia heterophylla, a ground cover species which supports sev- eral of the natural enemies of bagworms, in minimising build-up of M. plana.

Other bagworms: Wood (1968a) mentioned species of Pteroma, Clania and Amatissa as showing limited

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increases in Malaysia from time to time. Oiketicus kirbyi occurs in Costa Rica (Genty et al., 1978).

In Colombia and Ecuador, Stenoma cecropia (Stenomidae) has occasionally caused extensive damage. This species has a fixed bag, and eats by journeys from it while remaining attached by fibres (Genty, 1978). There are some natural enemies, but control by them is reported to be weak and, in serious outbreaks, aerial spraying with trichlorfon has been necessary for effective control.

12.2.5.3 Other caterpillars

Damage by caterpillars of other families is also reported from time to time. Homophylotis catori (formerly Chalconycles) (Zygaenidae), which has a life cycle of about 30 days, was reported as causing serious local damage in the Ivory Coast (Genty, 1968). Outbreaks followed the use of BHC against leaf miner. Pyrrhochaleia iphis (Hesperiidae) and Epimorius adustalis (Psychidae) have been reported from Congo (Frazelle and Buyckx, 1962).

In Malaysia, Wood (1968a) and Mariau et al. (1991) recorded a large number of leaf-eating caterpillars, few of which, however, have done any significant damage. Cutworms (Noctuidae), usually Agrotis sp., can cause damage in prenurseries and Spodoptera litura may strip the leaf epidermis in nurseries.

In America, colonies of mixed species of Lepi- doptera have characteristically developed on some plantations, and the method and timing of control has influenced the balance between species. Opsiphanes cassina (Brassolidae) did much damage as a leaf eater and was reported to be encouraged by carbaryl spraying, but reduced by using lead arsenate or B. thuringiensis (Rojas-Cruz, 1977). In the mixed colonies there have been species of Megalopyge (Megalopygidae), species of Dalceridae and Hesperiidae, and Herminodes insula (Noctuidae) in the spear leaf, and some Psychidae.

12.2.5.4 Grasshoppers

Bush crickets, long-horned grasshoppers or treehop- pers (Tettigonidae) are described as the principal pest of oil palms in PNG (Prior, 1988; Caudwell, 2000). Three species, collectively known as Sexava, are oil palm pests: Segestes decoratus, Segestidea defoliaria and

S. novaeguineae. Prior (1988) noted 80–90% defoliation by S. defoliaria. Prior listed a number of species parasitic on Sexava, but according to Caudwell (2000), experi- ence suggests that populations are not well controlled by natural enemies; once light damage is observed, this will steadily increase until severe defoliation has occurred.

Control: Bush crickets can be controlled by trunk injection of monocrotophos (Prior, 1988), but treatment must be in the early stages of an outbreak to be effect- ive. Caudwell (2000) described a census system based on the amount of visible leaf damage, rather than on pest numbers. Mass rearing and release of egg parasites was described by Prior (1988), but it seems that these methods do not give complete control, and further work on biological control methods is in progress (Kathirithamby et al., 1998; Caudwell, 2000).

12.2.5.5 Leaf miner, Coelaenomenodera lameensis (C. minuta, C. elaeidis)

This beetle is found on oil palms and, to a lesser extent, on coconut and Borassus palms throughout West and Central Africa. Serious attacks, causing widespread defoliation, have been reported from Ghana, Benin, the western part of Nigeria, Ivory Coast and West Cameroon, although for a long period the leaf miner was only reported from Ghana. There are numerous species of Coelanomenodera. Until 1980, the main oil palm pest was thought to be C. elaeidis, but then two species were recognised, with C. minuta thought to be the more important. It now appears that the pest is a third species, C. lameensis (Berti and Mariau, 1999). Although C. elaei- dis occurs on oil palms, it is not thought to be involved in outbreaks (R. Philippe, pers. comm., 2001).

Life cycle and damage: The method of feeding and life history were recorded in Ghana by Cotterell (1925). Very detailed studies have been made of the biology of this insect and of its control (reviewed by Mariau, 1976a). The life history in days is as follows: eggs, 20; larvae, 44; pupae, 12; adult to egg laying, 18; total, 94. The adults continue to live on the undersurface of the leaf for 3–4 months during and after laying eggs. The length of the life cycle accounts for the pest damage reappearing in some cases every 3 or 4 months.

A single female of C. lameensis may lay several hundred eggs; C. elaeidis is less prolific, perhaps explaining why outbreaks of that species do not occur (R. Philippe, pers. comm., 2001). The larvae, which grow to about 7 mm in length, are brown and their heads are squeezed into the thorax, their flattened bodies being transversely divided by deep furrows. They mine under the upper epidermis of the leaflets of palms of all ages except, normally, those below 3 years old in the field. The gal- leries are longitudinal, and in a severe attack the greater part of the leaf tissue will be destroyed (Plate XVIC). A single gallery mined by a larva to attain its full develop- ment measures about 15 cm in length and is 1 cm broad. Severely attacked palms have a typical appearance; the

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young leaves are green, being little attacked, while the remainder are grey–brown and withered, with desic- cated, rolled-in leaflets. Later, the withered laminae shatter, leaving the leaflet midribs only.

The pupae are found in the dead tissue of the leaves, and the adults, which are 4–5 mm long, emerge after about 12 days. The pupae are mobile and are found in the centre of the galleries. The adult emerges through the upper epidermis and shows a preference for migrat- ing to the higher leaves. These adults are pale yellow with reddish wing cases; they also do some damage, making grooves about 1 cm long on the leaflets. The female lays her eggs in a small cavity on the underside of the leaf.

The effect of damage on yield is similar to that of other pests which cause defoliation. Philippe et al. (1979) estimated that there was a 40% yield loss in the 2 years following an outbreak.

Control: The census method developed in the Ivory Coast (Mariau and Bescombes, 1972) involved count- ing of adults and larvae on a leaf between 25 and 30 (i.e. in the lower part of the canopy), with small and large larvae, nymphs and adults being recorded separately. The palms selected for counting are changed at each census round. Counting is done every 3 months when the number of larvae is below 10 and of adults below 1; monthly when the numbers are 10–20 and 1–3, and weekly if more than 20 and 3, respectively. When the lat- ter stage is reached treatment is considered necessary.

Spraying with Evisect (thiocyclam), using a tractor- drawn sprayer or a helicopter, or by fogging, was rec- ommended by Philippe (1990a, b). A single treatment was usually effective, but sometimes a second application 3 weeks after the first was needed. There was only a small effect on the population of the pollinating weevil, Elaeidobius kamerunicus, and there were no detectable residues of thiocyclam in palm oil. Trunk injection has also been recommended (Philippe and Diarrassouba, 1979); with suitable equipment, this is easily done over large areas (Sarjit, 1986).

Cotterell (1925) reported hymenopteran parasites of both the eggs and the larvae, as well as fungal parasitism. In the drier parts of the West African palm belt where leaf miner damage has been serious in some years, the attacks seem ordinarily to have been controlled by natural preda- tors, and resurgence has not occurred again until, for some reason, the parasite population has fallen below normal. Parasitism of Coelaenomenodera was studied in detail by Morin and Mariau (1974), Mariau and Morin (1974) and Mariau et al. (1978). The eggs are parasitised by the chalcid fly, Achrysocharis leptocerus, and by Oligosita longiclavata (Trichogrammatidae). There were three larval parasites: the eulophid flies Sympiesis

aburiana, Pediobius setigerus and Cotterellia podagrica. None of these was sufficiently numerous to have much limiting effect in outbreaks (Mariau et al., 1978), so a search has been made for possible parasites to introduce into the Ivory Coast for control of the pest. The eulophid wasp, Chrysonotomyia sp., was successfully introduced from Madagascar, but it failed to parasitise C. lameensis (Lecoustre et al., 1980). In Cameroon, Timti (1991) found that leaf miner attacks were fewer where Crematogaster ants were present, and suggested that the pest could be controlled by collecting these ants and distributing them in affected areas.

12.2.6 Stem pests of mature palms

12.2.6.1 Rhynchophorus species (Curculionidae): palm weevils

Rhynchophorus is a potentially lethal pest. In Asia and Africa its incidence on the oil palm is not very high; deaths have been noted in Africa where leaves have been cut abnormally short and wounding of adjacent leaf bases has resulted. In America, incidence may well be higher. Deaths from R. palmarum attack have been noted in young plantings within the grove areas in Bahia, Brazil, and the pest is quite frequently encountered on oil palms in other parts of the continent. Its greatest importance in America may be as the vector for the nematode that causes red ring disease (Section 12.1.6.6).

Distribution and description: Species of these large weevils are to be found attacking palms in all parts of the tropics. The larvae tunnel into the crown and trunk, and the palm may be killed. As pests of the oil palm the distribution of the more important species is as follows:

R. phoenicis

Africa

 

R. palmarum

America

Gru-gru beetle

R. ferrugineus

Asia

Red palm weevil,

 

 

red-stripe weevil

R. papuanus

Celebes, New

 

 

Guinea

 

The larvae attain a length of some 5 cm and are ovoid or rounded, legless and yellowish-white, with small, brown heads. The last abdominal segment is flattened and has brown edges carrying bristles. The cocoons of the pupae, constructed of concentrically placed fibres, extend to 8 cm in length and 3.5 cm in breadth. The adults show distinct specific differences but are usually about 4–5 cm long and 2 cm broad. Rhynchophorus phoenicis is black with two narrow longitudinal dark

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brown bands on the thorax; the wing cases have about a dozen longitudinal grooves. The underside of the body is light brown with diffuse black spots. Rhynchophorus ferrugineus is the common red palm weevil of the east; it is the species most commonly found in Sumatra and Malaysia (described under the synonym R. schach, and known as the red-stripe weevil). The oil palm has, how- ever, proved far less liable to attack than the coconut palm. Rhynchophorus ferrugineus is rather variable in length (2–5 cm) and is red–brown with a few irregular black spots on the thorax. The variety known as R. schach is black with a longitudinal red–brown line down the centre of the thorax. The American species, R. palmarum, is entirely black with a velvety thorax, slightly pro- longed at the base, and shiny grooved wing cases.

Life cycle and damage: Rhynchophorus weevils lay their eggs, which are 2–3 mm long, on cut or damaged surfaces of many palms. The eggs hatch in 3 days, and the larvae tunnel into the crown and trunk (Plate 12.8). The tissues around the growing point then begin to decay and the palm may be killed. The external symp- toms of attack have been described as similar to those of Fusarium wilt: the leaves show a gradually increasing chlorosis and fracture in strong winds.

The larval stage lasts for about 2 months and pupa- tion then occupies about 25 days, the larvae moving towards the periphery of the trunk to pupate. The whole life cycle lasts for less than 3 months. The weevil

Plate 12.8 Rhynchophorus palmarum larva found in a ‘spear rot’ palm in Nicaragua. (B.J. Wood.)

more commonly breeds in the stumps of a felled palm field, newly cut stumps being preferred. Oryctes and Rhynchophorus species are often present in a plantation at the same time. Wounds made by Oryctes adults give a means of Rhynchophorus infection, while Rhynchophorus damage will provide conditions suitable for Oryctes larvae (Hartley, 1988; Zulnerlin and Fatah Ibrahim, 1999).

Control: Effective control of Rhynchophorus attack is not easy. In the first place, wounding of the palm must be avoided and the petioles must not be cut close to the trunk. Secondly, all dead or felled palms should be destroyed within the period of the beetle’s life cycle. Measures for the control of Oryctes and other large beetles will help to reduce the incidence of Rhynchophorus. Mariau (1968) described various pre- ventive and curative measures, including hooking the larvae from their tunnels with the aid of a wire.

The most promising approach is to trap the adult weevils. Initially, traps were baited with sugar-cane, pineapple, banana or palm tissue, but pheromone-baited traps are more effective. Oehlschlager et al. (1992) described the aggregation pheromone of R. palmarum. This compound, ‘rhynchophorol’, is released by male weevils, and attracts others of both sexes to the site of release. Pheromones have also been described for

R. phoenicis (Gries et al., 1993), R. ferrugineus (Hallett et al., 1993) and various other species. Additional attractants include ethyl acetate and ethyl propionate, which are produced by damaged palm tissue, and the most effective traps were baited with the pheromone and palm tissue pieces (Gries et al., 1994). The work on pheromones is summarised by Giblin-Davis et al. (1996). Chinchilla et al. (1995) described the use of pheromone-baited traps to capture R. palmarum and hence to control red ring disease.

Several parasites of Rhynchophorus species have been recorded. Moura et al. (1993) described a tachinid fly,

Paratheresia menezesi, which parasitises R. palmarum in Brazil.

12.2.7 Root pests of mature palms

12.2.7.1 Oil palm root miner

The caterpillar of the moth Sagalassa valida (Bracho- didae) has been found mining in the roots of oil palms in several South American countries including Colombia, Ecuador, Peru and Brazil. Up to 80% of the root system may be destroyed, and attacked palms may die.

Life cycle and damage: The female moth has a wing span of 2.1 cm, the male 1.8 cm. They live in the under- growth and among the cut palm leaves in the interline, and their dull colour blends with that of the withered material. The position of egg-laying has not been

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439

observed, but it is presumed that it is in moist material such as lichens, mosses or humus at the base of the palm. The young larvae, which are no more than 1 mm long, penetrate the primary roots immediately after hatching, but can also move through the soil to attack roots some distance from the point of hatching. They at first eat the external part of the root, leaving the central cylinder intact; this partial destruction stimulates the production of new branch roots. Older larvae grow to 2 cm long, and cause complete destruction of the tis- sues of the roots in which they mine (Genty, 1973).

In areas of attack it has been noted that the number of caterpillars increases with the age of the palm, and in some palms 50–80% of the root system has been destroyed, including old and recent damage. Some attacked palms fall over (Genty, 1981). The amount of damage is highly variable, but tends to be greater on the edge of a plantation near the forest or near to rivers and streams.

Control: The possibility that sudden wither is associ- ated with damage by this caterpillar has already been mentioned (Section 12.1.6.4). Genty (1977) considered that the extent of the damage done to the roots in itself justified treatment, and stated that a generalised yellowing of the leaves may be due solely to Sagalassa. He recommended routine checks by examination of one hole, 40  40  50 cm deep, at the foot of one palm/ 20 ha every 6 months. If more than 20% of the primary roots are attacked more intensive checks are done, and if 20% attack is still found then an insecticide should be applied around the bases of the palms. Treatment with endrin (now banned) was always followed by rapid regeneration of the root system.

12.2.8 Pests attacking fruit and bunches

In addition to the insects listed below, rats do consider- able damage to fruit bunches (Section 12.3.1).

12.2.8.1 Oil palm bunch moth

Distribution: Tirathaba rufivena (Pyralidae: Galleri- inae; formerly T. mundella) is widespread in Malaysia and Indonesia and can reach epidemic proportions, especially in young areas.

Life cycle and damage: Eggs are laid in the bunches, especially those overripe or rotten, and in inflores- cences or bunches lying on the ground. Caterpillars bore into developing fruit or feed on the surface of ripening fruit. They are sometimes found tunnelling into the base of a spear leaf. They are light to dark brown and grow to 4 cm before pupating as dark brown pupae inside the bunch.

Control: Wood and Ng (1974) recommended spray- ing with endosulphan, but Basri Wahid et al. (1991) found that Thuricide (B. thuringiensis) was more effect- ive than endosulphan, and cheaper than other effective insecticides. Two larval parasites have been identified: a chalcid wasp, Antrocephalus sp., and an ichneumon wasp, Venturia palmaris, of which the latter was more com- mon and appeared to be the more promising for possible biological control (Ng, 1982).

12.2.8.2 Eupalamides cyparissias

(Castiniidae): oil palm bunch miner

This pest (formerly known as Castnia daedalus) has done serious damage to bunches in Guyana, Surinam and Peru.

Life cycle and damage: The butterfly, which has a wing span of 17–21 cm, lays its eggs on unripe bunches. The larvae grow to 13 cm in a period of about 8 months, passing through 14 larval stages. The insect then pupates in the leaf bases for a period of 30 days. The lar- vae bore into the peduncles and bunches, causing rotting, and also into the stem. There is a high mortality from wasp and fly parasitism (Korytkowski and Ruiz, 1980).

Palms are attacked as soon as they start bearing, and provided harvesting is complete the larvae will be detected in the bunches and a measure of control obtained (Huguenot and Vera, 1981). Mariau and Huguenot (1983) described methods of estimating populations of larvae of different stages with the object of initiating control measures before the dangerous later larval stages are reached.

Control: Various control methods have been tried. Van Slobbe (1983) found injection of monocrotophos and carbofuran ineffective, but application of granular carbofuran in the spear region was successful. Huguenot and Vera (1981) recommended trichlorfon or carbaryl.

12.2.8.3 Demotispa neivai

Following the extended planting of the oil palm in Colombia, Demotispa neivai (previously Pseudimatidium or Himatidium) was reported from the Magdalena val- ley, and it has become a pest of the oil palm in all parts of South America, although not usually causing serious losses. A new species, P. elaeicola, was discovered on the Pacific coastal plain near Calima.

Life cycle and damage: The adult of D. neivai meas- ures 5  3 mm and is at first white but rapidly becomes shiny brown with fine longitudinal lines along the wing cases. Single eggs are laid. The larvae are more flattened than the adult and their feet are short and withdrawn; they are at first translucent, later turning dull red, and

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reach 7  4 mm. The pupae are brown and otherwise resemble the larvae. The insect is found on the under- surface of leaves, but the main point of attack is the fruit (Figueroa and van den Hove, 1967). Most damage is done by the larvae, which nibble the exocarp begin- ning at the apex. A fungus then develops at the point of attack and the exocarp becomes lignified and grey.

It has been estimated that a heavy attack leads to a loss of 7–9% of oil production, but that losses from the more usual mild attacks are, in spite of the alarming appearance of the bunches, negligible (Genty and Mariau, 1973). Young plantations are more vulnerable.

Control: The pupae of D. neivai are parasitised by

Tetrastichus sp. and Psychidosmiera sp., but the amount of control exercised by these appears small. Ants are considered to play a more important role in limiting the population. Disbudding of young palms was recommended as a control measure by Mariau (1976b), if incidence exceeded 30%. Only if the attack becomes severe (a general attack of more than 70% of the palms, or more than 10% attacked heavily) is insecticide treatment thought necessary (Genty and Mariau, 1973).

12.3 MAMMALS AND BIRDS AS PESTS

12.3.1 Rats

Distribution: In the past, the most important mamma- lian pest of oil palms in Malaysia was Rattus tiomanicus (formerly R. jalorensis), the Malayan wood rat. Young plantings may be infested by the rice field rat, R. argen- tiventer, but R. tiomanicus is found in virtually all estab- lished oil palm plantations. The black, house or roof rat (R. rattus diardii) has also become an oil palm pest in parts of Malaysia (Wood et al., 1988), and may now be almost as common as R. tiomanicus. Wood and Chung (1990) found that R. r. diardii occurred in areas where R. tiomanicus had developed warfarin resistance (see Control, below), and suggested that R. r. diardii could only compete with R. tiomanicus where the latter species had been weakened by the rapid evolution of resistance. Other rat species found in Asian oil palm fields are

R. exulans and the bamboo rat, Rhyzomis sumatrensis. The genus Rattus only occurs in West Africa as

introduced species in populated areas, but several other species are found in plantations. Dasymys incomtus,

Lemniscomys striatus, Lophuromys sikapusi and Uranomys ruddi were recorded in the Ivory Coast by Bellier (1965) and Brédas et al. (1968).

A long-term study showed that the population of R. tiomanicus in a Malaysian plantation without rat

control fluctuated slowly between about 200 and 500 rats/ha (Wood, 1984). Wood and Liau (1984b) showed that R. tiomanicus was potentially capable of doubling in number every 46 days. However, in the absence of control measures, the actual population remained quite stable over long periods, rarely exceeding about 500 rats/ha. The reasons for this were not clear, but exter- nal environmental variables were deemed unlikely to be controlling factors, and predation appeared to depend on rat numbers, rather than the reverse. It seemed that there was some intraspecific mechanism, with popula- tion pressure affecting breeding success or survival of young rats. This needs to be remembered when con- sidering possibilities for biological control by predators (see below).

Damage: Wood (1976b) estimated that a population of 300 rats/ha would consume about 480 kg of meso- carp/year, representing a loss of about 240 kg oil (Plate 12.9). This is about 5% of a good plantation yield, but the estimate ignored the loss of detached fruit. Taking account of detached fruit, Liau (1990) estimated total losses at up to 10% of production.

In an untreated area, palms showing fresh damage ranged from 0 to 39%, with an average over 10 years of 11% (Wood and Liau, 1984a). Wood (1976b) showed that there was a relationship between amount of fresh damage and rat population, and considered that 5% fresh damage indicated an economically damaging infestation. However, the correlation between amount of damage and rat population is not very strong, either for fresh damage or for detached fruit removed (Liau, 1990).

Young palms are sometimes attacked, probably mainly by R. argentiventer. If necessary, palms can be protected with wire-netting collars, as against the ‘cutting grass’ (see below), although the collar must be turned in at the top (Wood, 1968a).

Control: Poison baiting has been the main method of control, using anticoagulants, but there has been much interest in recent years in the possibility of biological control by owls.

Detailed instructions for baiting were given by Wood and Nicol (1972). Baits consisted of warfarin in cubes of maize and other ingredients, solidified with wax. The baiting procedure was a simple one: one bait per palm was distributed, with replacement of missing baits at 4-day intervals, until acceptance fell below 20%. With this system, rat populations could be reduced to negligible levels by four or five rounds of baiting. The recommendation was to repeat baiting every 6 months, although reinfestation could take over a year. Rein- festation appeared to be partly from outside, and partly

Diseases and Pests of the Oil Palm

441

Plate 12.9 Fruit damaged by rats; the apical part of the meso- carp, and the kernel, have been eaten (from Corley, 2001).

from survivors (Wood, 1970). The former source is reduced by systematic baiting over large areas. Costs of control by baiting are typically equivalent to between 10 and 25% of the value of the lost oil (Wood, 1977; Chung and Balasubramaniam, 2000).

Warfarin baiting worked well for many years, but warfarin resistance eventually developed (Wood et al., 1990) and is now widespread in Malaysia. Where resist- ance has developed, newer anticoagulants such as brod- ifacoum and bromadiolone have proved effective. With increasing labour costs, Chung and Balasubramaniam (2000) investigated alternatives to the replacement bait- ing method. They found that the replacement system remained the cheapest, even with high labour costs, but if sufficient labour was not available, then placing sev- eral baits per palm, or one large bait, in a single round was an effective alternative.

Recommendations for control of rats in Africa (IRHO, 1976) involved warfarin baiting on the same basis as described by Wood and Nicol (1972), together with wire-netting collars to protect young palms.

Biological control: In recent years there has been much interest in the barn owl, Tyto alba, for biological control of rats in oil palms in South-east Asia. Numbers of barn owls have increased enormously in Malaysia, following the expansion of the oil palm industry. In 1951, Glenister (1951) classified the species as ‘very rare in Malaya’, its having been recorded only three or four times. By the 1980s, Lenton (1985) described it as common. Lenton (1980) showed that numbers were limited by lack of nest sites, and designed a nest box;

these have been used successfully to encourage breed- ing in plantations. By 1989, Smal (1990) found one owl per 17 ha in one oil palm plantation, and one breeding pair per 8.5 ha in another.

Duckett (1982) summarised work by Lenton, showing that barn owls consumed large numbers of rats, which could comprise as much as 98% of their diet. It was calculated that a breeding pair of owls and their young would consume 1200–1500 rats/year (see Duckett and Karuppiah, 1990). With one pair per 8.5 ha (see above), this is equivalent to about 160 rats/ha per year.

With an uncontrolled population of up to 600 rats/ha, there must be some doubt as to the degree of control that owls will exercise, and Wood (1985) noted that there is no well-documented example of a predator exercising continuing control over a vertebrate pest. Often, the predator population depends on the prey population, rather than vice versa; thus, snakes disappeared from oil palm plantations after rat control by intensive baiting started (Wood, 1985). We have already noted that rat populations stabilise at a certain level, despite having the potential to multiply exponen- tially. If a proportion of the population is removed by a predator, reproductive success may simply increase to restore the equilibrium population level. Alternatively, the population may stabilise at a lower level. Key ques- tions then are: what is the equilibrium population level under owl predation, and is the amount of damage caused by that population economically acceptable?

Numerous authors have shown that owl numbers can be increased by providing nest boxes, but actual data on

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control of rats are sparse, and those studies that have been done have not taken account of the large fluctu- ations in rat population that can occur in the absence of any control measures (Wood, 1984). Heru et al. (2000) estimated rat populations by trapping in plantations in Indonesia. Numbers trapped diminished from 100/ha in the year owls were introduced, to 20/ha 2 years later. They describe these figures as ‘population density’, but Smal (1990) estimated that in his study actual popula- tions were about three times the numbers trapped. The relationship will, clearly, depend on the method and efficiency of trapping. However, if the rat popula- tion in Indonesian estates was reduced to as low as 60 rats/ha, that may be below the range of normal population fluctuations (Wood, 1984), suggesting that owls had an effect. In Malaysia, Smal (1990) found that rat numbers decreased after the owl population built up, but rat numbers also fell to a similar level in fields without nest boxes. This could have been because the owls hunt over a large range, well beyond the fields with nest boxes, but it could also have been a natural decline caused by some other factor. Chia et al. (1995) estimated populations of up to 400 rats/ha under owl predation in one estate, comparable to levels without control (see above).

Several comparisons of fresh damage levels before and after the introduction of owls have been made. It might be argued that the amount of damage is what matters, not the number of rats, but fresh damage is only a rough indicator of rat population, and is not the only damage done. Other rodents have been shown to change their habits when predators are about (e.g. Abramsky et al., 1996), and it is possible that, when owls are present, rats spend more time in frond piles, consume more detached fruit and do less damage to bunches still on the palm. If feeding is mostly on detached fruit, then when considering the economics of rat control, one must ask whether such fruit would have been recovered if it had not been taken by rats (see Section 10.4).

Wood (1976b) considered that 5% fresh damage indicated an economically damaging population level. This has been interpreted as meaning that less than 5% damage is acceptable, but Wood used the 5% figure as an indication of a need for control by baiting. After baiting, damage should be reduced to zero, and will remain at that level for several months, so the average amount of damage over time would be well below 5%.

Duckett and Karuppiah (1990) found fresh damage on 15–20% of palms before owls were established and on 8–15% after establishment. Smal (1990) found that fresh damage was reduced to about 3% after establish- ment of owls in one estate, but remained above 7% on

another. Ho and Teh (1997) found that damage in a 500 ha block of palms decreased to below 5% by the third year after establishment of owls, and remained low for the next 5 years, without baiting. Hoong H.K. (2000) found that, after the introduction of owls to Sabah, fresh damage in estates averaged 5%, compared with over 10% a decade earlier. In smallholdings, the comparable figures were 24% and 10%. In general, therefore, it appears that the amount of damage done when rats are controlled solely by owls may be less than without control, but is close to, and sometimes above, the threshold for baiting recommended by Wood (1976b). However, Adidharma (2002) claimed that fresh damage level was reduced from 30% to 0.14% after introduc- tion of owls to a 14,000 ha plantation in Indonesia.

Smal (1990) and others have suggested that good control might be achieved by a combination of barn owls and limited baiting. If this is to be done, it is essen- tial that owls eating poisoned, but still living rats should not be affected by the anticoagulant. There are various reports that owls are not affected by warfarin, but Lee (1995) found that warfarin and the second generation anticoagulants were all toxic to owls. The second gener- ation anticoagulants were more toxic, but doses of warfarin taken by rats, and hence by owls, were higher. More work is needed if an integrated system using both owls and baiting is to function effectively, parti- cularly on the choice of anticoagulant where warfarin resistance occurs.

Conclusion: Control of rats with anticoagulant baits is well established and cost-effective. However, as Chia et al. (1995) noted, the fact that planters will abandon a proven method in favour of barn owls, which are ‘at best unproven’, is a clear indication of the attraction of biological control. Owl populations are easily built up by provision of nest boxes, and they consume large numbers of rats. It appears that the equilibrium rat population in the presence of owls may be lower than in their absence, but the rats still do some damage. Smal (1990) calculated that construction of nest boxes was much cheaper than baiting, but noted that owls could not eliminate rats completely. In most studies, the amount of fresh damage has remained close to the 5% level that Wood (1976b) considered to indicate the need for baiting. For a proper comparison, the cost of this residual damage must be included in the costs of rat control by owls. Reliable trials would need to cover large areas, because of the hunting range of the owls, and would have to include an uncontrolled area, to show natural population fluctuations. Until such trials are done, the effectiveness and the economics of biological control of rats by owls remain uncertain.

Diseases and Pests of the Oil Palm

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12.3.2 Other mammals

The ‘cutting grass’, Thryonomys swinderianus, is com- mon in Africa, and young areas planted near to the for- est are particularly at risk from its devastating attacks. Protection against this pest using wire collars is described in Section 9.1.4.3. Porcupines (Hystrix brachyura) attack young palms on forest margins, gnawing through to the bud. Wire collars are not effective; zinc phosphide baits with palm oil in cassava root (Wood, 1968a) and chem- ical repellants (Chandrasekharan and Edmunds, 1976) have been employed.

Elephants have done great damage to young plantings in South-east Asia, systematically uprooting rows of newly planted seedlings. A ditch, 2.5 m deep and wide, may be an effective deterrent; though expensive, the cost is easily justified if there is a known risk of elephant incursion (Wood, 1977). Wild pigs damage or kill young palms, and monkeys occasionally pull up seedlings. Liaw (1983) described trapping methods used in Sabah for the control of these pests. In Indonesia, electric fences have proved effective for excluding pigs (Schmidt, 1986).

Squirrels (Callosciurus spp.) are occasionally trouble- some in Asia, eating the mesocarp and sometimes attack- ing nursery plants.

12.3.3 Birds

The long-tailed parakeet (Psittacula longicauda), the blue-rumped parrot (Psittinus cyanurus) and the Malay lorikeet (Loriculus galgulus) have all been troublesome in Malaysia. Most destructive is the long-tailed parakeet, which feeds in flocks of up to 30 birds, carries away ripe fruit from the bunch and tends to scatter it about half- eaten. Such damage can be distinguished from rodent damage by the single beak groove in the fruit. The other species feed close to the bunch and do not scatter the fruit. Shooting is the only control known and with the long-tailed parakeet this does not seem to have been very effective (Wood, 1968a). However, with the expan- sion of the area under oil palms, most estates are now contiguous with other cultivated land, rather than adja- cent to the parakeet’s forest habitat, so their importance as a pest has diminished (B.J. Wood, pers. comm., 2001).

The American black vulture (Coragyps atratus) has become a serious pest in Brazil, Colombia, Honduras and elsewhere. In some countries these birds are protected by law as useful scavengers, and special permission must be obtained to shoot them. This course has been adopted in Colombia. In India, Dhileepan (1990) estimated that birds consumed up to 2.8 t FFB/ha per year. He sug- gested that cages could be used to protect bunches.

In West Africa the village weaver (Ploceus cuculatus) may be locally troublesome, stripping the leaflet lam- inae from a wide area to make nests in adjoining trees. It is usually necessary to fell the nesting trees to dis- perse the birds.

12.4 INSECT VECTORS OF DISEASES

As well as causing direct damage, some insects play an important role as vectors of disease. Leptopharsa gibbi- carina (Hemiptera: Tingidae) appears to be the vector of Pestalotiopsis in leaf wither, and control of this insect has checked the spread of the disease (Section 12.1.5.2). A bug, Recilia mica (Delphacidae), has been implicated in the transmission of blast disease (Section 12.1.4). The weevils Rhynchophorus palmarum and Metamasius hemipterus are vectors of the nematode that causes red ring disease (Section 12.1.6.6). Two species of Sogatella (Homoptera: Delphacidae) have been shown to trans- mit a coconut disease similar to dry bud rot (Julia and Mariau, 1982).

Several insects have been associated with sudden wither (Section 12.1.6.4). Sagalassa valida was first sug- gested, then the hemipteran Myndus crudus (Haplaxius pallidus), but Lincus lethifer and L. tumidifrons (Hemip- tera: Pentatomidae) now appear the most likely can- didates.

12.5 PESTS OF OTHER COMPONENTS OF THE OIL PALM AGROECOSYSTEM

12.5.1 Pests attacking pollinating weevils

Since its introduction to oil palm-growing areas outside Africa, the pollinating weevil Elaeidobius kamerunicus (discussed further in Section 2.2.2.5) appears generally to have thrived. There have been occasional sugges- tions that poor pollination was due to pest or disease attack, but there is no convincing evidence for such effects. Liau (1985) reviewed what was known at that time, and little seems to have been added since.

The main predators on the weevil in the Far East are undoubtedly rats. These consume large numbers of lar- vae, destroying the old male inflorescences in the process, and Liau (1985) showed that rats grew more rapidly on a diet supplemented with weevil larvae. Basri Wahid and Halim Hassan (1985) considered that rat populations, and the amount of damage done, had increased since the introduction of the weevil. Chiu et al. (1985) estimated that up to 80% of weevil

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larvae might be eaten by rats, but noted that the weevil population remained high enough to ensure good fruit set.

Kang and Zam (1982) detected, and eradicated, two species of parasitic nematodes in weevils imported to Malaysia. There have been more recent reports of wee- vils parasitised by nematodes in Malaysia, though (R.A. Syed, unpub.; Rao and Law, 1998), and Poinar et al. (2002) described a new species, Elaeolenchus parthenonema, which reproduces asexually and is an internal parasite of E. kamerunicus. Rao and Law showed that parasitised weevils had shorter life expectancy and lower reproduc- tive rates than unparasitised. They considered that these effects, combined with low numbers of male inflores- cences (the weevil breeding sites) during periods of high oil palm sex ratio, could lead to the poor fruit set which has sometimes been observed in parts of East Malaysia.

12.5.2 Pests attacking legume cover crops

Wood (1976a) and Liau (1979) reported that leaf-eating caterpillars, grasshoppers and cockchafers may do

considerable damage to legume cover crops. Perhaps equally importantly, though, Wood also noted the exist- ence of ‘chronic’ pests, particularly the bug Chauliops bisontula. The occurrence of this pest in Malaysia, and the debilitation that it can cause, may explain why cover crop growth is often very weak in that environment. Liau (1979) reported a trial in which cover crop pests were controlled by insecticide spraying. The planted legume species made up a consistently greater propor- tion of the total ground cover biomass in the sprayed than in the unsprayed plots, indicating that the pests were reducing the vigour of the legumes. In the latter, more than half of the biomass consisted of weeds by 11 months after sowing.

Liau did not recommend insecticide spraying of the cover crop as a practical measure; spot spraying of weeds with herbicide is clearly more environmentally benign. However, the trial did help to explain why so much time and effort has to be spent on managing and main- taining the cover crop in Malaysia, whereas in most countries Pueraria phaseoloides, and the other leguminous species used, grow strongly in young palm plantings, easily suppressing most weeds.