PATHOGENIC VARIABILITY IN MYCOSPHAERELLA-FIJIENSIS MORELET, CAUSE OF BLACK SIGATOKA IN BANANA AND PLANTAIN

Sixty-three strains of Mycosphaerella fijiensis Morelet obtained from various Musa hosts in a range of different countries and localities we re inoculated to juvenile plants of a standard set of Musa genotypes. Individual strains had consistent but different patterns of pathogenic ity on the host set. Some genotypes (SF215, II-249 ('Saimea'), 'Grande Naine') were susceptible to practically all isolates. Others, e.g., ' T8' and 'Calcutta', were resistant to some isolates but susceptible to others. Strains from Papua New Guinea and the Pacific Islands exhibit ed a wide range of pathogenic diversity. Some strains from those regio ns were pathogenic on juvenile plants of genotypes commonly used as so urces of resistance in breeding programmes ('Calcutta', 'Paka', and 'P isang Lilin'). These strains may threaten resistant progeny derived fr om those genotypes. Analysis of pathogenicity of strains collected ove r 4 years in Rarotonga, Cook Islands, has confirmed field observations that 'Paka' resistance is no longer effective on that Island. 'Paka' virulence has been found to be widespread in strains from the Pacific Islands and Papua New Guinea. Juvenile plants of 'Yangambi' (AAA), gen erally considered to be resistant to M. fijiensis, were susceptible to strains which were virulent on 'Paka'. The availability of pure cultu res of strains with differential virulence on specific Musa genotypes will facilitate the analysis of the inheritance of resistance to M. fi jiensis.