Pests & Diseases

DISEASES

Cocoa is affected by a range of pests and diseases, with some estimates putting losses as high as 30% to 40% of global production.

Witches´ Broom

History, distribution and impact

Moniliophthora perniciosa is a fungus responsible for Witches’ Broom disease. During the last century the fungus spread throughout all of South America, Panama and the Caribbean, causing great losses in production. The most visible effect can be seen in Brazil where the introduction of the disease in the region of Bahia caused a decrease in production of almost 70% during a period of 10 years.

Disease symptoms

The fungus attacks only actively growing tissue (shoots, flowers and pods) causing cocoa trees to produce branches with no fruit and ineffective leaves. The pods show distortion and present green patches that give the appearance of uneven ripening.

Epidemiology

The life cycle of the fungus is synchronized with the phenology of the host. One of the most influential factors for the adequate reproduction of the fungus is water. Basidiospores are released at night and are related to the level of humidity (=80%) and favourable temperature (20-30ºC). The spores are capable of being disseminated locally by water and convection currents and over long distances by wind.

Quarantine

Basidiospores have a short viability period and are sensitive to light and drying but are produced in vast numbers (each basidiocarp can produce 2-3.5 million spores). Humans are considered the crucial factor in long-distance dissemination. The pathogen is also spread in infected seeds or budwood.

Disease resistance

Host resistance is recommended as the best option for economic and sustainable control. During the 1930s, selections were identified showing resistance in Trinidad. As a result, Trinidad Selected Hybrids were developed and widely planted during the 1950s. However, more aggressive strains of the pathogen in other countries made these selections ineffective. CEPLAC (Brazil) is currently working on new molecular techniques such as genetic linkage maps and quantitative trait loci to develop new resistant varieties.

Fungicides

Various fungicides have been tested showing various results. New compounds and chemicals, which activate the host plant’s defences, may offer a more effective and economical control.

Cultural practices

Phytosanitary pruning is the only effective means of control of Witches’ Broom. Complete removal of all infected material is advocated, but it is an impossible task because hidden inoculum sources always remain.

Prospects

The new techniques developed in the project financed by ICCO/CFC in cooperation with international institutions identified individuals with accumulated genes leading to reliable resistance to the disease. The information evaluated during the project was more precise and scientists were able to produce more resistant varieties than expected at the beginning of the project. The first impact of the project outcome can be seen in Brazil, where the cocoa industry has re-gained confidence and production has been increased significantly

Frosty pod Rot

History, distribution and impact

Frosty Pod Rot is caused by the basidiomycete Moniliophthora roreri. It is found in all north-western countries in South America. First reports of the disease date back to the end of the 19th century, where its aggressive effects caused devastation in Colombian and Ecuadorian cocoa plantations. The fungus has now spread all over the Latin American region, causing significant losses in production, even resulting in the abandonment of cocoa farms.

Disease symptoms

The fungus infects only actively growing pod tissues, especially young pods. The time from infection to the appearance of symptoms is about 1-3 months. The most outstanding symptom is the white fungal mat on the pod surface.

Epidemiology

The large amount of spores produced (44 million spores per cm2) and the genetic variability endows the fungus with considerable adaptability. The dry, powdery form of spores allows the fungus to be dislodged by water, wind or physical disturbance of the pod. Disease incidence varies with cultivar, pod age and rainfall. Generally the greatest production is when rainfall is high.

Quarantine

Since spores are the only means of infection, all materials used must go through intermediate quarantine facilities. Application of fungicides is required when using seeds for reproduction as well as when treating budwood.

Disease resistance

All cocoa species seem to be susceptible to this disease. Some varieties have shown a degree of resistance and field screening has identified clones with low disease severity and incidence. Genotypes which produce their pods during the dry season (unfavourable for the pathogen) escape the disease.

Fungicides

The use of copper and organic protectors has proved to reduce the incidence of the disease. Systematic fungicides such as Flutolanil have been found effective, although the use of agrochemicals is not economically sustainable in view of the low prices of cocoa.

Cultural practices

Crop sanitation involving the removal of infested pods is the main method of control of the disease. This activity has to be done with extreme care due to the fact that healthy pods can be infected during the process.
Prospects

Frosty Pod accounts for about 5% of total annual crop loss. The socio-economic impact of the disease in Latin America could be enormous as many farmers rely on the cocoa crop as a source of income. The use of copper fungicides will continue to be the most common control measure for the disease in the short term. A more integrated management strategy using biocontrol agents and resistant planting material is envisaged in the future.

Phytophthora pod rot

History, distribution and impact

Pod Rot, also know as Black Pod, is caused by the fungus Phytophthora spp. Three fungal species of the same genus are responsible – P. palmivora, P. megakarya and P. capsici. The P. palmivora causes global yield loss of 20-30% and tree deaths of 10% annually. P. megakarya is the most important pathogen in Central and West Africa, known as the most aggressive of the Pod Rot pathogens. P. capsici is widespread in Central and South America, causing significant losses in favourable environments.

Disease symptoms

Obvious symptoms are the rotting or necrosis of pods. Pods can be attacked at any stage of development, and the initial symptoms are small, hard, dark spots on any part of the pod. Internal tissues, including the beans, are colonized and shrivel to form a mummified pod.

Epidemiology

Under humid conditions a single mummified pod infected with P. palmivora can produce up to 4 million sporangia which can be disseminated by rain, ants, flying insects, rodents, bats and contaminated pruning material. In the case of P. megakarya, sporulation is usually more abundant. The soil borne phase of the P. megakarya disease cycle causes root infection maintaining a reservoir of inoculum that releases zoospores into the soil surface water. P. megakarya does not survive in mummified pods but can survive in infected debris for at least 18 months, while P. palmivora survives less than 10 months in the soil.

Quarantine

Efforts are being held to prevent the movement of P. megakarya from West Africa to other cocoa growing regions. Cocoa germplasm should go through intermediate quarantine facilities when multi-breeding projects are developed.

Disease resistance

Breeding for resistance offers the best long-term management strategy. Reliable screens for resistance are been developed and DNA markers could aid breeding programmes. Another approach is to seek out healthy individual trees among the great diversity of genotypes on farms under high natural disease pressure.

Fungicides

Protectant sprays of copper based fungicides, combined with the systematic fungicide metalaxyl under high disease pressure, at three or four weekly intervals are frequently recommended. Some controls involving the injection of the trunk with cheap inorganic salt and potassium phosphonate have proven to be effective against P. palmivora in some producing regions.

Cultural practices

Modification of farm management practices to optimize shade and aeration through appropriate spacing and pruning to reduce surface wetness should be effective. Frequent and complete harvesting, sanitation and appropriate disposal of pod mummies, infected pods and pod husks can reduce the disease.

Prospects

Understanding how to achieve and maintain healthy soils on cocoa farms is fundamental to sustaining higher yields and lower levels of disease. A healthy soil is one that contains high organic matter and plant nutrient content, abundant and diverse microbial activity, good drainage and physical structure. Cultural practices are useless against P. megakarya, therefore many farms have been abandoned in West Africa. An integrated global breeding programme is in progress to identify potential sources of resistance and other options for management.

Vascular-streak dieback (VSD)

History, distribution and impact

This disease is caused by the fungus known as Oncobasidium theobroma. It was first distinguished in the 1960s in Papua New Guinea when it caused heavy losses of trees in mature plantations. The disease has spread ever since and is found in South East Asia, causing major losses in large commercial plantations in Malaysia.

Disease symptoms

The initial characteristic symptoms are the chlorosis of one leaf on the second or third flush behind the tip. The fungus may spread internally to other branches or the trunk, usually causing death of the tree. When an infected leaf falls during the rainy season, hyphea may emerge from the leaf scar and develop into a basidiocarp, evident as a white, flat, velvety coating over the leaf scar and adjacent bark.

Epidemiology

The formation and forcible discharge of basidiospores occurs mainly at night. The spores are dispersed by wind although effective spore dispersal is limited by high humidity. The basidiocarps only develop when leaf fall occurs during wet weather. They have a short life-span and release spores only at night and when the basidiocarps are sufficiently moist.

Quarantine

Quarantine measures to restrict transport of the fungus by humans are crucial in reducing inter-regional spread of the disease. Transmission of the disease through fermented and dried seeds has not been demonstrated.

Disease resistance

Selection for a degree of resistance to the disease of survivor plants from Papua New Guinea has been identified and used for propagation. The use of resistant material brought from various regions produces resistant hybrids that are been distributed in Papua New Guinea. Resistant types still become infected but at lower levels and the pathogen grows more slowly. The resistance has been sustained and lasted from 1963 to the present time.

Fungicides

Protective fungicides are unlikely to be effective against this disease since infection occurs mainly during the wet season and thus the product is washed away. The systematic fungicide propiconazole painted on the stems of young seedlings or applied as a spray was effective in nursery conditions.

Cultural practices

Pruning diseased material about 30cm below the discoloured xylem prevents further expansion of infection and reduces inoculum levels. Opening the canopy and control of shading to increase aeration and insolation of the foliage are important. The raising of planting material under plastic cover and away from infected plantations ensures that the material used for reproduction will be disease-free.

Prospects

The integrated management techniques developed in South East Asia have proved to be a useful tool to control VSD. There has been an upsurge in the incidence of VSD in Papua New Guinea due to the wetter conditions of recent years. The introduction of hybrid cocoa, including the Upper Amazon and Trinitario germplasm, has resulted in a great genetic diversity in the region which has proved to have considerate levels of resistance.

In order to control the effect of the pests and diseases described above, a project entitled “Cocoa Germplasm and Conservation: a Global Approach” was developed by IPGRI and implemented in 10 cocoa producing countries across the world under the supervision of ICCO and with main financing from CFC. The aim of the project was to develop and distribute improved cocoa varieties that would be resistant to pests and diseases and produce high yields, leading to sustainable cocoa production.

INSECT PESTS

Mirids

History, distribution and impact

Mirids are the major insects that affect cocoa worldwide. In Ghana, cocoa mirids have been recognized as a serious pest since 1908 due to their devastating effect. The most common species in Ghana and West African countries are Distantiella theobroma and Sahlbergella singularis. In South-East Asia the Helopeltis spp. is responsible for the damage related to mirids while Monalonion species are present in South and Central America. Mirid damage alone, if left unattended for three years, can reduce yields by as much as 75%.

Symptoms, biology and ecology

Cocoa mirids pierce the surface of cocoa stems, branches and pods, killing the penetrated host cells and producing unsightly necrotic lesions. Mirids feeding on shoots often result in the death of terminal branches and leaves, causing dieback. Mated female mirids lay up to 60 eggs that are embedded in the bark of stems or inside the pod husk. Pests usually occur on trees exposed to sunlight since such trees tend to bear more fresh shoots and pods. Although the insect is attracted to trees exposed to sunlight, after locating their source of food they inhabit shady areas on trees. Some indigenous plants grown with cocoa have been identified as alternative hosts for some species of mirids.

Resistant varieties

Some clones with low water content in their stems are unattractive to mirids. Preliminary laboratory and field screening of cocoa genotypes have identified locally available genotypes in West African countries as well as international clones which are less preferred by mirids.

Pesticides

The use of organochlorine insecticide has proved to be effective in Ghana. The current recommended insecticides are Imidacloprid, Actellic/Talstar and Promecarb. Insecticides are applied as foliar spray four times per year at monthly intervals using motorized mist-blowing machines. An alternative approach is to apply two sprays of persistent or systemic insecticides a year on mature cocoa under closed canopy. Reduced insecticide use also allows populations of natural enemies to increase and provide a more suitable environment for biological control.

Cultural practices

Shade and canopy management should be designed to achieve a balance between mirid control, flowering and black pod management. Alternative hosts should not be used as shade trees on cocoa farms.

Biological control

The application of Integrated Pest Management (IPM) is a key tool in controlling mirids. The black ant (Dolichoderus thoracicus) has been used in some farms as a control measure. Another ant (Oecophylla smaragdina) has also been used although this one can be aggressive.

Prospects

Mirids are well known for their sporadic distribution, cryptic habits and highly damaging effects, which make them difficult and extremely important to control. The use of conventional insecticides has proved to be the most effective method of control in West African countries. Ghana is working on the development of comprehensive IPM strategies that could be adopted throughout the region.

Cocoa pod borer (CPB)

History, distribution and impact

The Cocoa Pod Borer (CPB), also known as Cocoa Moth, is caused by the insect Conopomorpha cramerella. It was first noted in 1841 as a serious threat, causing vast losses in the cocoa industry during the 1890s and 1900s. CPB now affects almost all cocoa producing provinces in Indonesia. By 2000, CPB had infested 60,000 ha, inflicting losses of US$ 40 million per year. The spread of CPB along with the decline in the price of cocoa led to decreases in production in Malaysia.

Symptoms, biology and ecology

CPB attacks both young and mature cocoa pods. A common symptom of infested pods is unevenness and premature ripening. Infestation of young pods results in heavy losses because the quantity and quality of the bean becomes seriously affected. A CPB moth is 7mm long and brown in colour with a white strip on the forewings. The female lives for about 5-7 days and lays 100-200 eggs on the surface of the cocoa pod. The larval period is about 14-18 days, then 5-8 days before hatching as a moth.

Resistant varieties

Efforts are focused on developing genotypes with harder walls in their pods. Higher mortality from larval instar is found in genotypes with either thicker or harder endocarp. Attempts are being made to screen selections for resistant material from the diversity of genotypes planted in Sulawesi.

Pesticides

Improved control using relatively small amounts of contact pyrethroid or carbamate insecticides, applied to the undersides of lower branches, keeps the CPB population below economic damage levels. However, this is used as a last resort due to the high cost of pesticides and the low prices of cocoa.

Cultural practices

Sanitation practices involving the complete harvesting of ripe or damaged pods, burying of pod husk, placenta, rotten pods and all harvest remains are recommended. Regular pruning of the cocoa canopy to less than 4 m in height is also good practice. Pod-sleeving with plastic bags also reduces attacks of CPB. Pods should be sleeved when they are about 8-10 cm long and the sleeves should be left throughout the pod maturation period.

Biological control

Ants such as the black ant (Dolichoderus thoracicus) and the weaver ant (Oecophylla smaragdina) are recommended. The fungus Beauvaria bassiana has also been found to infect larvae and pupae, causing a 100% death rate. The use of traps with synthetic pheromones or female pod borer moths can control the attack if enough males are caught, thereby interrupting their reproduction cycle.

Prospects

Cultural control is the most likely way of reducing CPB. This relies on the maintenance of well pruned trees kept to a height low enough to allow the collection of all pods. Long-term control may be improved by side-grafting or replanting with resistant clones. Biological control is another area that deserves more investigation, particularly into using biopesticides and natural enemies.