Genetic diversity in the coffee wilt pathogen (Gibberella xyla-rioides) populations: Differentiation by host specialization and RAPD analysis

摘要

Gibberella xylarioides causes a vascular wilt disease, tracheomycosis, in almost all coffee species in Africa. The disease is endemic on Arabics coffee in Ethiopia and re-emerged as a major threat to Robusta coffee in Congo, Uganda and Tanzania. The population structure of the pathogen was studied in recently isolated and historic strains employing pathogenicity tests and RAPD analyses. In infection tests, the isolates from Arabica coffee plants were pathogenic only to seedlings of Coffea arabica with significantly varying degrees of aggressiveness, but incompatible with seedlings of C. canepbora. In contrast, the Robusta strain was specifically compatible with seedlings of C. canephora without inducing any infection symptom in all C. arabica cultivars. This result is the first cross inoculation evidence proving host specialization of G. xylarioides populations to the two commercially important coffee species. There existed also highly significant differences among cultivars, isolates and cultivars-isolates interactions. In the RAPD-PCR analysis, the Ethiopian Arabica isolates clustered into a homogeneous population, but were distinctly polymorphic to strains from C. canephora and C, excelsa. The historic Arabica strain was only slightly differentfrom the recent collections illustrating little genetic change in the population structure over the last three decades. The results of both host-pathogen interactions and RAPD-PCR markers corroborated the existence of host specialization into at least two pathogenic forms within G. xylarioides populations. Thus, two formae speciales, namely G. xylarioides f. sp. abyssiniae (anamorph: Fusarium xylarioides f. sp. abyssiniae) for the fungal strains attacking only C. arabica and G. xylarioides f. sp. canephorae (anamorph: F. xylarioides f. sp. canephorae) pathogenic to C. canephora and C. excelsa are proposed. This subdivision enables to design effective coffee wilt management strategies, develop resistant cultivars/lines and formulate further breeding programs towards each population group.