Dieback disease caused by Fusarium and Lasiodiplodia species is a major threat to
cocoa production in Ghana and elsewhere in West Africa. Current recommendations
involve insecticide application to control mirid bugs whose feeding punctures provide
entry points for these fungi. Little is known about the true identity of the causal
pathogens of this disease. Earlier work implicated F. decemcellulare as the causal
agent and more rarely L. theobromae (Cotterell, 1927; Crowdy, 1947). A total of 117
single spore fungal cultures was established from diseased cocoa stems imported from
Ghana. On morphological grounds cultures could be designated as either Fusarium or
Lasiodiplodia spp. The Fusarium cultures exhibited inter-isolate variability with
respect to macroscopic appearance and macro-conidium morphology, suggesting the
presence of more than a single species. The isolates were further characterised by
PCR amplification and sequencing of the ITS region of rDNA and comparison with
authentic reference cultures. Thirty-seven Fusarium isolates were identified to twenty
F. chlamydosporum, nine F. solani and four isolates each of F. oxysporum and F.
proliferatum. The thirty-six Lasiodiplodia isolates were identified to two species,
twenty-seven L. pseudotheobromae and nine L. theobromae. In pathogenicity tests, F.
chlamydosporum, F. oxysporum, F. proliferatum, F. solani and L. pseudotheobromae,
previously unknown as pathogens of either cocoa or any member of the Malvaceae,
caused significant wilting and dieback in Amelonado seedlings similar to that
observed in the field. All isolates exhibited optimal growth at 30 ºC on PDA. Disease
incidence in 29 and 15 cocoa germplasm lines in the laboratory and greenhouse,
respectively, showed reproducible differences in their reaction to necrotic lesion and
dieback infection. LCTEEN 37/F was one of the most susceptible genotypes. CATIE
1000, T85/799 and MXC 67 were the most tolerant and could be used in cocoa
breeding programmes for resistance to dieback
