DNA RESTRICTION-FRAGMENT-LENGTH-POLYMORPHISMS IN MYCOSPHAERELLA SPECIES THAT CAUSE BANANA LEAF-SPOT DISEASES

Restriction fragment length polymorphisms (RFLPs) of leaf spot pathogens of bananas, Mycosphaerella spp., were examined with low-copy nuclear DNA probes isolated from a M. fijiensis cosmid library. The two related fungi, M. musicola and M. fijiensis, displayed different RFLP patterns and hybridization intensities, suggesting considerable interspecific genetic divergence. The synonymy of M. fijiensis var. difformis with M. fijiensis was supported because RFLP patterns were similar and did not reflect morphological variations between these fungi. Fifty-seven isolates of M. fijiensis with different geographic origins were assayed for RFLPs. The highest level of genetic diversity in this pathogen was found in Southeast Asia, the region where it probably originated according to the chronological spread of M. fijiensis in the world. In other areas, isolates formed genetically homogeneous groups specific for each region (Africa, Pacific islands, and Latin America). Such geographical differentiation suggests that there was limited introduction of M. fijiensis into each of these regions.