Cocoa has been described as a “virtuous crop”. There is an increasing appreciation of its value for: land rehabilitation, enrichment of biodiversity (of previously cleared land) and provision of sustainable incomes in less developed regions. Like other crops though, it can be attacked by a number of pest species including fungal diseases, insects and rodents – some of which (e.g. frosty pod rot and cocoa pod borer) have increased dramatically in geographical range and are sometimes described as “invasive species”.
Fungal diseases are a principal constraint to world cacao production and on a global scale the greatest losses are result from black pod rots – caused by Phytophthora spp. (see below). Two basidiomycete fungal diseases – witches’ broom and frosty pod rot – pose a special threat to livelihoods in Latin America. Black pod rots currently cause the greatest loss of production, but estimates of severity perhaps underemphasise the potential importance of frosty pod rot: Moniliophthora roreri.
Some cocoa problems have a world-wide distribution, others are restricted to individual cocoa growing regions in: the Americas, Africa and S.E.Asia.
Black pod: Phytophthora spp:
Phytophthora spp. belong to the water moulds (Oomycete) group of diseases, that cause black pod in cocoa. They are not fungi, but are controlled with products that are often called “fungicides”.
In Africa, P. megakarya has become a major invasive disease, having spread from central region westwards to Ghana and the Ivory Coast.
Invasive Moniliophthora diseases:
Latin American problems but a global threat …
Frosty pod rot is an invasive disease which, originally identified in Ecuador in 1917, spread rapidly to other countries in Latin America during the 1970s. Producing large quantities of powdery spores, it has a high potential for transmission and thus further extending its range. Infested plantations may suffer dramatic yield loss, leading to neglect of trees by farmers and complete loss of production.
The fungal pathogen that causes witches’ broom disease has now been identified as a close relative of frosty pod rot, and they have now been placed in the same genus: Moniliophthora
Other cocoa pod diseases
CSSV is transmitted by mealybugs (Pseudoccidae) including Planococcoides njalensis and species in Planococcus, Phenacoccus and other genera.
Mistletoe in cocoa provides a habitat for a certain species of ants that “farm” mealy bugs, (other ant species may be beneficial). The presence of mistletoe also favours infestation by mirids.
Vascular streak die-back (VSD):
Fungal diseases in the soil, such as Ceratocystis fimbriata and Roselinia spp. can cause relatively sudden tree death. The latter have been important when associated with cocoa germplasm collections.
Ceratocystis wilt of cacao, caused by a host-specialized form of the fungus, has been locally important in Latin America, where it is believed native and called mal de machete. Its importance in Brazil has been recognised increasingly since 1998 and has been associated elsewhere with drought, with South Bahia experiencing reduced rainfall in recent years. Xyleborus beetles are attracted to the diseased trees and bore into the branches. The frass from beetles is pushed to the outside of stems as a light, powder and contains viable inoculum of the fungus, which may be spread by wind or rainsplash.
a. West African Miridae (capsids)
Sahlbergella singularis (left): geographically the more widespread species. Right: Distantiella theobroma.
The greatest damage is to the tree itself, with destruction of growing shoots (above left) and whole trees may be killed.
In some parts of S.E. Asia, other species in this genus such as Helopeltis theivora (= H. theobromae) and H. antonii are serious enough to spray regularly with insecticides.
Cocoa pod borer (CPB)
Zeuzera spp include Z. coffeae: larvae of leopard moths (Cossidae) that bore into trunks of many tree species. The galleries formed are especially damaging to young cocoa, often causing the snapping-off of smaller branches.
In Papua New Guinea (PNG) one of the major pests are two species of longicorn beetle (Cerambicidae) Glenea spp. (G. aluensis from New Britain shown here). Longicorn larvae are often recognisable by the enlarged, flattened segments just behind the head capsule.
In PNG and Tuvalu, striking, flightless weevils (Curculionidae) belonging to the genus Pantorhytes are primary cocoa pests. Often tunnelling in numbers around the jorquette and branches, they may be also responsible for ring barking and killing whole branches.
Adult and larva in gallery of Pantorhytes pluteus taken in New Britain, PNG. Both Pantorhtes and longicorn beetles (left), besides causing damage to trees themselves, may introduce diseases such as Phytophthora palmivora cankers: possibly resulting in tree death.
Rodents and other vertebrate pests
|Further information on insect pests can be obtained from:
Entwistle P F (1972) Pests of Cocoa. Longman tropical Science Series, London. 779 pp.
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