7.2.2 Diseases

Black pod

In many growing seasons, the black pod pathogen Phytophthora megakarya causes the greatest crop loss in West Africa, the world’s most important cocoa growing region. 

Fungicides, in combination with cultural control methods, are widely used for control of the disease:
 

• Cultural methods are essential: poor aeration within the crop canopy may encourage the disease, so thinning the canopy can help. Fungicides will only work well in combination with appropriate tree height and canopy management facilitate pod inspections.
• Weekly phytosanitation to remove diseased pods and ideally remove them from the field to reduce the risk of them becoming a secondary source to infection.
• It is important to remove soil on cocoa trunks (soil tunnels are often built by ants on the surface of cocoa trunks). This eliminates two sources of disease: spores carried in infested soil and those carried by the ants themselves.
• Apply appropriate fungicides using correct application methods.
  • Copper compounds have contact action – so good coverage is essential.
  • They can be supplied singly, or they may be mixed with …
  • Systemic compounds (Table 4.2) including: (a) phenylamides (metalaxyl and benalaxyl), which have long been widely available and are cost effective [1], (b) more recently CAA fungicides (group H5) such as dimethomorph and mandipropamid.
• Make sure that it is worth applying a pesticide. Establish that:
  • the infestation is above an appropriate action threshold
  • it is not too late to spray (i.e. if too much damage has already been done – as in this severe attack of P. megakarya black pod disease).
  • With infestations such as the one shown here, the only useful control measure would be to remove and destroy the infected pods and bury them if possible, in order to reduce the release of spores.
• Soil health and general good crop management are essential. Soils contain nutrients for the cocoa trees, but also can harbour the pathogen. Soils with high organic matter and good drainage help prevent inoculum splashing and spreading in puddles of water.
• The hyperparasite, Trichoderma asperellum appears to be the most promising biological control agent found to date and was previously available as a commercial product in West Africa, but this no longer appear to be the case.
• Cankers can develop on branches and the main trunk when infection occurs through the pod stalks or from the soil at the base of the trunk. The diseased tissue should be scraped back and treated with fungicide before the infection kills the branch or trunk [2].
• In South -East Asia, Phytophthora trunk cankers have been successfully treated by trunk injection of potassium phosphonate.