Cocoa: Pests and Diseases

Cocoa is very susceptible to pests and diseases right from the moment the seeds are sown.

All the plant parts including the pods are affected by various pests (Conway, 1971; Pang & Syed, 1971; Shah, 1976 and Mainstone, 1978) and diseases (Liu & Liew, 1975; Turner & Shepherd, 1978 and Varghese, 1985). Some of them eg. rodents, pod borer, Helopeltis and Phytophthora are capable of causing very high crop losses while VSD has destroyed many young plantings, debilitated mature cocoa bushes and reduced yield in Sabah recently.

Currently the most important pest disease in Malaysia are the moth pod borer, Conopomorpha cramerella and Vascular Streak Dieback (VSD) caused by a fungus Oncobasidium theobromae . The former is confined to Sabah and Sarawak. VSD is present throughout Malaysia.

Conopomorpha cramerella (cocoa pod borer)

The cocoa pod borer was first reported in Sabah in late 1980. The pest if left to build up is capable of destroying the entire crop. Several control measures have been tried to contain the pest including rampassen (i.e. total removal of all pods susceptible to attack) with varying degree of success (Wood, 1980; Day, 1983; and Mumford, 1984).

Currently, no single control measure is entirely satisfactory on its own. A combination of control measures (integrated control) including cultural, biological and chemical methods usually produces better results.

The most common control methods currently practised are:-

  • clean and frequent harvesting and breaking of pods as soon as possible and destroying/bagging/burying of husks to prevent pupation.
  • selective spraying of moth resting sites i.e. on the undersides of the branches inclined at less than 45ofrom the horizontal and
  • sleeving of young pods to prevent the moths from laying eggs.

Mass rearing of parasitic wasps, Trichogrammatoidea bactrae fumata Nagaraja for release against pod borer eggs and the use of synthetic pheromone for trapping male moths and to disrupt mating are reported to be promising and are still being evaluated. Lately, the Commonwealth Institute of Biological Control (CIBC) in collaboration with EMPA/MCGC have also started a search for exotic natural enemies.

Oncobasidium theobromae (VSD)

The disease causes dieback of canopy and can kill the young bushes. The problem is more serious during the establishment phase. Mature cocoa can usually survive the attack, but varying degrees of yield losses may be expected depending on the severity of the disease and the susceptibility of the cocoa.

Vascular Streak Dieback (VSD) is present throughout Malaysia. Lately, the disease became very serious in Sabah, destroying numerous nurseries and young plantings. Even the mature cocoa bushes were debilitated resulting in vast crop losses.

Byrne (1976) reported 25 to 40% losses in PNG. Taylor and Chong (1983) reported 30% losses when the VSD infested bushes were severely pruned in Lower Perak.

Numerous control measures including pruning, shade adjustment, manuring, fungicide spraying and isolation of nursery and new plantings have been tried. However, the disease was so virulent in 1984/85 that widespread planting failures and high crop losses were reported despite desperate attempts made to control the disease with all the control measures known. It is hoped that 1984/85 were exceptional years in Sabah and that the disease would not be as virulent in the “drier years” when the environment is not as conducive for VSD.

1) Chemical control

A wide range of fungicides have been evaluated in Malaysia and PNG. While none of the fungicide trials carried out in PNG showed any promise, Chung (1983) reported that Bitertanol (Baycor 25% wp) at 0.05% a.i. + 0.05% Agridex sprayed to slight run-off at 14 days interval gave complete protection to nursery seedlings.

Bitertanol is reported to be a protectant and has no curative properties.

Musa & Tay (1984) reported that Benomyl (Benlate), Pyracarbolid and Triforine totally inhibited the growth of VSD mycelium in vitro (lab test). Benomyl applied as soil drench was translocated to the leaves with a residual effect of 1 month. However the efficacy of the fungicides has not been evaluated in the field and therefore cannot be recommended on the basis of actual field trial results.

It must be pointed out here that Benomyl has been reported to be not effective against VSD in the field. Chung (1983) reported that Benomyl did not provide any control at all. Instead the VSD incidence of the Benomyl treated seedlings was higher than the control. More field trials on Benomyl are needed.

Varghese (1985) reported that Triadimenl, Paropiconazole and PP969 are promising in invitro screening.

Currently, Bitertanol appears to be the only fungicide proven to be effective against VSD, although there were some conflicting reports from Sabah. Its application is mainly restricted to the nursery at present. As it has no curative effect, the chemical must be applied in the first instance as soon as the seed starts to emerge followed by a second spray when the first leave emerges and the application continued regularly for as long as the protection is needed.

2) Isolation/Barrier

Under normal conditions, VSD fungal spores do not travel for more than 200 m. An isolation belt of more than 200 m would normally reduce the inoculum potential sufficiently to reduce the chances of infection.

Isolation is most useful for nursery. Isolated nurseries generally have less VSD problem.

In very high inoculum areas, covering the nurseries with ultra-violet light resistant polythene sheets has been reported to confer protection in PNG. The main effect is to keep the leaves dry and hence not conducive to fungal spore germination.

3) Disease avoidance

Avoid planting cocoa during maximum risk period i.e. when the weather is unusually wet for a prolonged period and when the inoculum potential is very high.

Presently, there is insufficient information to define high risk periods accurately. However the following factors taken together may be considered to constitute a high period:-

  • in very wet years when rainfall is more than 2000 mm per year.
  • When there is continuous wet weather for 4 or more days.
  • When very high incidence of VSD is noted and hence high disease pressure.

4) Pruning

Pruning to control VSD is a much debated and controversial subject in Malaysia and deserves a special mention here. The subject has been dealt by numerous authors (Kean & Turner, 1971; Jayawardena et al. 1978; prior, 1980 and Taylor & Chong, 1983).

In Papua New Guinea, it was reported that pruning to remove all VSD tissues and 20 cm beyong the streaking at 2-3 weeks intervals to keep the disease inoculum low is able to control the disease in young cocoa i.e. before the canopy closes over (Odonohue, Pers Comm.)

However, others noted that if pruning is too severe, it could cause more harm and wondered if it is better to leave the bushes alone to allow them to grow out of the disease. The trouble is, not all of them can. At the same time the diseased bushes provide a constant source of inoculum which could result in alarming outbreaks in conducive environments. To derive the best results from pruning, the disease must be controlled in its early stages when it is possible to keep the inoculum at a very low level through regular and frequent prunings.

5) Disease resistance/tolerance

Selection for disease resistance/tolerance is probably the best long term solution for VSD control.

The cocoa industry in PNG was nearly destroyed by VSD at one time. However, with the introduction of VSD resistant/tolerant clones, VSD is now no longer a problem in PNG. Tan (1983) reported that the resistance is polygenic and is not likely to break down easily.

6) Rehabilitation

Varying degree of success have been reported by budding VSD infested bushes with VSD tolerant clones, mainly in Lower Perak.

The approach appears promising.

7) Cultural practice/nutrition

Good cultural practices and nutrition can go a long way to help the bushes to combat the disease. Healthy and vigorous bushes are certainly in a better position to withstand the disease infection and if infected will have a greater chance of recovery.

Manipulation of shade to ensure good aeration and reduction of humidity can also reduce the chances of infection. However, it is important that young cocoa should not be over exposed by excessive shade removal.

Helopeltis, rodents and black pod

Apart from VSD and pod borer, Helopeltis spp (Theivora and theobromae) and rodent pests (mainly rats and squirrels) are also capable of causing very high crop losses in excess of 90%. (Tan, 1974; Mainstone, 1978 and Han, 1982).

The main rodent pests in Malaysia are rats and squirrels. Han and Subash (1980) found that both the rats and squirrels consumed 2.5 to 3.0 pods per feeding in a cage trial. This works up to about 30 to 36 kg of dry bean per rodent per year assuming one feeding per day. The same authors estimated the population of rats and squirrels in an estate in Bagan Serai could reach as high as 100 to 300 and 30 to 90 per hectare respectively. It is therefore not surprising to find the entire cocoa crop lost to the rodents.

The most effective way to control rats in cocoa plantation is baiting with anti-coagulant poison (Friend, 1971; Ooi, 1977 and Han 1982)

Squirrels may be controlled by a combination of trapping with ordinary drop-door rat traps using jack fruit baits (Artocarpus heterophyllus) and shooting. A combination of baiting for rats and trapping/shooting for squirrels is able to reduce the pod damage by rodents to less than 5%. (Wanless, Pers. Comm.)

As Helopeltis initial outbreaks are usually in localized patches, the pest is best controlled at this stage with 2 consecutive sprays of insecticides such as Gama BHC or Propoxur at 10-14 days intervals. To achieve the best results, the pest must be monitored and detected by an Early Warning System and sprayed selectively to prevent major outbreaks (Wills, 1984)

Phytophthora black pod disease is a potentially very serious disease in Malaysia. Shepherd et al. (1977) reported that up to 5% of the crop could be lost to the disease in Lower Perak. The losses are probably higher in the wetter areas such as in Jerangau and certain areas in Sabah. Elsewhere in the world, crop loses of as high as 75% have been quoted by Gregory (1974).

A combination of cultural practices aim at reducing the inoculum pressure through frequent harvesting and proper disposal of diseased pods, pruning to improve aeration and finally chemical control when the infections have reached beyong 5-10% if advocated by Varghese (1985).

McGregor (1982) reported that metalaxyl and cuprous oxide were effective in controlling the black pod disease in Papua New Guinea.

Minor pests/diseases

The other common but less serious pest and diseases recorded in Malaysia include the following:-

  1. Colletotrichum leaf spot and phytophthora blight in the nursery
  2. Civets, monkey, Dichrocrocis punctiferalis Porthesia similis and Conopia spp . pod pests.
  3. Botryodiplodia theobromae brown pod disease.
  4. Zuezera coffeae Endoclita hosei Xyleborus spp and Inderbella biciblaga stem/bark borers.
  5. Phytophthora stem canker, pink disease (Corticium salmonicolor) and Marasmius thread blight diseases on the stem.
  6. A wide range of Coleopteran, Lepidopteran and Orthopteran leaf eating pests
  7. White root (Rigidoporus lignosus) and brown root (Phellinus noxius) diseases

From the foregoing, it is not difficult to see that pest and disease control is a very important aspect of cocoa estate management.

Reference 
Ooi L.H. and Chew P.S. 1985. Some important agronomic and agricultural practices in cocoa  estates. TDMB Plantation Management Seminar, Kuala Trengganu

Note: The full list of references quoted in this article is available from the above paper.

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