PATHOGENIC SPECIALIZATION AND GENETIC VARIATION IN THE JAPANESE STRAINS OF FUSARIUM OXYSPORUM F. SP. MELONIS
F NAMIKIl, T SHIOMIl, K NISHIl, T KAYAMURAl and T TSUGE2
l Kyushu National Agricultural Experiment Station, 2421 Suya, Nishigoshi, Kumamoto 861-11, Japan; 2 Graduate School of Bioagricultural Sciences, Nagoya university, Chikusa, Nagoya 464-0 1, Japan
Background and objectives
Fusarium oxysporum f. ;sp. melonis causes Fusarium wilt of muskmelon (Cucumis melo). This forma specialis has been divided into races based on pathogenicity to a set of differential cultivars within C. melo. An understanding of the occurrence and distribution of such pathogenic variants is important in the development of adequate and effective methods of disease managements. In this study, we collected 41 strains of F. oxysporum f. ;sp. melonis from different locations in Japan and characterized them for pathogenic variation using 48 cultivars of muskmelon, oriental melon (C. melo var. makuwa) and oriental pickling melon (C. melo var. conomon), including race differential cultivars proposed by Risser et al. . Furthermore, we estimated genetic similarity among strains on the basis of FOLR DNA fingerprint comparisons.
Results and conclusions
Pathogenic variation among 41 Japanese strains of F. oxysporum f. ; sp. melonis was analysed by pathogenicity tests using cultivars of muskmelon, oriental melon and oriental pickling melon. Based on pathogenicity to two muskmelon cultivars, Amus and Ohi, and an oriental melon cultivar, Ogon9, 41 strains were divided into three groups, which completely corresponded with Risser's races, 0, 2 and 1,2y. To further characterize pathogenic variation within the forma specialis and races, strains were assayed for pathogenicity to 42 additional cultivars from muskmelon, oriental melon and oriental pickling melon. All strains of race 1,2y were pathogenic to all the cultivars. Strains of race 0 were divided into six variants different in pathogenicity to three muskmelon cultivars, and strains of race 2 were also classified into six variants based on differences in pathogenicity to two muskmelon cultivars and an oriental melon cultivar. Genetic variation among strains was analysed by DNA fingerprinting with four repetitive DNA sequences, FOLRL to FOLR4. Thirty-six fingerprint types were detected among 41 strains by pooling results of fingerprinting with four probes. Cluster analysis showed distinct genetic groups correlating with races: the fingerprint types detected,in each of race 2 and racel,2y were grouped into a single cluster, and two distinct genetic groups were found in race 0. However, pathogenic variation detected within race 0 and race 2 could not be differentiated on the dendrogram. These results indicate that races of F. oxysporum f. ;sp. melonis in Japan have DNA polymorphisms, and that there are intra-race, pathogenic variations which are not correlated with the nuclear markers examined.
1. Risser G, Banihashemi Z, Davis DW, 1976. Phytopathology 66, 1105-06.