Cocoa, Theobroma cacao, bean is the source of all forms of world’s chocolates. One major drawback to its production is a disease called black pod disease. The black pod disease is caused by many species of Phytophthora, the fungi-like organisms known as Oomycetes. The black pod disease can cause up 80% yield loss if not properly managed.
Global Distribution and Impacts
Black pod disease is widely distributed throughout all cacao growing areas in the world and is ranked as the number one disease of cacao, with annual estimates of global crop losses of about 30% but up to 90% in some locations. This equals to or greater than 900,000 metric tons of commercial cacao (Saul-Maora et al., 2003). The impact is severe in some regions of West Africa including Nigeria, and Central Africa, which accounts for some 60 to 70% of the world’s cocoa bean production (Thevenin et al,. 2005).
Disease Development and Spread
The black pod pathogen survives spores or hypha in soil, infected pods or plant residues during winter. At the onset of the rainy season, it will germinates and produces zoospores. The zoospores are released in free water and spread to susceptble plant parts – such as flowers, and healthy pods – by rain-splash from litter, soil, sporulating pods or infected stems. The ineffective propagules can also be spread by ants.
Infection Process and Symptoms
On landing on surface of cherelles (young flowers) or pods, zoospore will germinate and form germ tubes which will invade the tissue either via stomata or (to a less extent) direct penetration via the exocarp of the pod. The first symptoms will appear 2 to 3 days after this as brown lesions that will develop quickly into a large brown rotten spot. Usually beans will be affected by the infection in immature pods, may be less or not affected in mature pods.
Copper-based fungicides. Although this is reasonably effective, the high cost of chemical control poses a serious challenge to peasant farmers who produce over 50% of the worldwide production. Metalaxyl and red copper oxide are the most effective fungicides commonly used to control black pod disease.
Genetic resistance. This is the most effective approach. However, progress in breeding has been hampered by a lack of effective screening methods, diversity of Phytophthora species causing the disease, and inadequate information on the types and components of resistance. Generally Amelonado types of cocoa have greater resistance to black pod disease than Amazon, Trinitario and Criollo.
Cultural practices. Include creating light shades by planting limited or regulated number of cocoa tree, pruning, and remover of the watershoots. Diseased pods are also removed as often as possible on regular basis and properly burnt or buried.
Integrated management that combines all the management options discussed above is the most effective against the disease.
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