Epidemiology of vascular‐streak dieback of cocoa

SUMMARY Inoculation studies provided further evidence that Oncobasidium theobromae is the cause of vascular‐streak dieback of cocoa and that the fungus can penetrate undamaged, soft leaves. Spores are forcibly ejected from basidia at night and dispersed by wind. They are rapidly killed by exposure to normal day‐time conditions and require free water for germination. Spore dispersal is probably very limited under plantation conditions and disease spread occurs along a steep gradient into new plantings. Disease increase in young plantations was found to occur at a rate of 0.31 per unit per month; in one block 100% of seedlings were infected and 55% were killed within 33 months of planting. The disease was most damaging in seedlings younger than 18 months old, although there was evidence that it also caused substantial yield loss in mature susceptible cocoa. Sporophore production and infection rate were correlated with rainfall incidence. The unusual grographical distribution of the disease and aspects of its control are discussed.