Pathogen diversity and host resistance in dieback disease of cocoa caused by Fusarium decemcellulare and Lasiodiplodia theobromae

Abstract

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.