Genetic diversity and pathogenicity of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> isolated from wilted Welsh onion in Japan

Thirty isolates of Fusarium oxysporum from wilted Welsh onion plants were examined for their diversity in nucleotide sequences of the ribosomal DNA (rDNA) intergenic spacer (IGS) region and for pathogenicity with regard to five Welsh onion cultivars. Phylogenetic analysis based on the IGS sequences revealed polyphyletic origins of the isolates and a relationship between phylogeny and pathogenicity; low virulence isolates differed genetically from those with high and moderate virulence. Mating type analysis revealed that all F. oxysporum isolates were MAT1-1 idiomorphs, suggesting that the pathogens may be clonal in the fields examined.