PATHOGENIC AND GENETIC-VARIATION IN THE JAPANESE STRAINS OF FUSARIUM-OXYSPORUM F SP. MELONIS

Pathogenic variation among 41 Japanese strains of Fusarium oxysportum f. sp. melonis was analyzed by pathogenicity tests with muskmelon, ori ental melon, and oriental pickling melon cultivars. Based on pathogeni city to muskmelon cvs. Amus and Ohi and oriental melon cv. Ogon 9, 41 strains were divided into 3 groups that corresponded completely to Ris ser's races 0, 2, and 1,2y. To further characterize pathogenic variati on within the forma specialis and races, strains were assayed for path ogenicity to 42 additional muskmelon, oriental melon, and oriental pic kling melon cultivars. All strains of race 1,2y were pathogenic to all cultivars tested. Strains of race 0 were divided into six variants ba sed on differences in pathogenicity to three muskmelon cultivars; stra ins of race 2 also were classified into six variants based on differen ces in pathogenicity to two muskmelon cultivars and one oriental melon cultivar. Genetic variation among strains was analyzed by DNA fingerp rinting with four repetitive DNA sequences: FOLR1 to FOLR4. Thirty-six fingerprint types were detected among forty-one strains by pooling re sults of flngerprinting with four probes. Cluster analysis showed dist inct generic groups correlated with races: the fingerprint types detec ted in each of races 2 and 1,2y were grouped into a single cluster, an d two distinct genetic groups were found in race 0. However, pathogeni c variation detected within races 0 and 2 could not be differentiated based on the nuclear markers examined.