Pests and diseases of cocoa

Pests and diseases cause heavy losses in cocoa production, making cocoa estates uneconomic in both the original home of the plant, South America, and the new centre of world cocoa production, West Africa.

The most important pests of cocoa in West Africa are capsid bugs; they feed on the young new growth, holding up the natural growth of the tree and preventing the development of pods. The extensive use of insecticides to control capsids and mealy bugs, the vector of virus diseases, has led to a disturbing increase in other insect pests, notably a pod miner and a shoot borer.

The most serious problem facing the West African cocoa industry is the menace of a complex of virus diseases referred to as swollen shoot. So far, some fifty distinct viruses have been distinguished as part of the swollen shoot virus complex. Many of these isolates are littoral, the time taken to kill the trees varying from three to six years. No method is known of curing an infected tree, so that there is no alternative to the destruction of infected trees in the control of the disease. The incidence of the problem has increased rapidly over the last 40 years. In 1936, when work on swollen shoot began, the outbreaks were limited but scattered over a large area. The outbreaks grew larger and fresh sites became infected in succeeding years. In the eastern region of Ghana the output of dry cocoa fell from 116,000 tons in 1936-7 to 38,000 tons in 1955-6, and now cocoa production in much of the region has been wiped out. Cutting out first started in 1941; and in 1948, following the advice of a Commission of Enquiry, a general policy of cutting out infected trees was adopted in West Africa. This policy led to 63 million trees, 10% of the total, being cut out in the period 1946-57 in Ghana alone. In Nigeria from 1946-50 a total of 1,500,000 trees were cut out; but thereafter large areas were abandoned as being too heavily infected. Although the campaign has been reasonably effective it has proved politically unpopular especially with farmers and is liable to interruption.

Another important cocoa disease is black pod, which, it has been estimated, reduces the world production by 7%, but in some areas, such as Nigeria and the Cameroons, losses may be as high as 90%.

The most damaging disease of cocoa in South America is Witches' broom disease, a fungal disease, which has completely destroyed whole crops. The original home of the disease was the Upper Amazon Valley. From there it has spread outwards to the neighbouring South American countries - Peru, Ecuador, Colombia, Venezuela and the Guianas. In the 1920s it practically wiped out cocoa growing in Surinam, Trinidad and Ecuador. It also infiltrated the West Indies, reaching Trinidad in 1928, Tobago in 1939, and Grenada in 1948.