VIEW ARTICLE

Ecology and Epidemiology

Population Structure and the Relationship Between Pathogenic and Nonpathogenic Strains of Fusarium oxysporum. T. R. Gordon, Assistant professor and staff research associate, Department of Plant Pathology, University of California, Berkeley 94720; D. Okamoto, Assistant professor and staff research associate, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 82:73-77. Accepted for publication 14 August 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-73.

One hundred isolates of Fusarium oxysporum were collected from soil with a history of Fusarium wilt of muskmelon in a 35 m² area within an agricultural field in the San Joaquin Valley of California. Twenty-nine strains from this collection, each representing a different vegetative compatibility group, were found to be nonpathogenic to muskmelon in greenhouse seedling tests. Each of these 29 strains was examined for polymorphisms in mitochondrial (mt) DNA by probing restriction digests of total DNA with cloned fragments from the mt genome of F. oxysporum f. sp. melonis. Changes in length (insertions or deletions) and restriction sites, relative to the reference strain of F. o. melonis, were inferred from the number and size of mtDNA restriction fragments that hybridized to Pst1 fragments from the reference strain. Thirty-seven changes were scored as present or absent in each strain. There were six unique character combinations (haplotypes) among the 29 nonpathogenic strains, all of which differed from F. o. melonis. One mtDNA haplotype was associated with 14 different vegetative compatibility groups. Parsimony analysis revealed a distant relationship between F. o. melonis and the mtDNA haplotypes associated with nonpathogenic strains. However, two of the nonpathogenic strains were more distantly related to other nonpathogens than they were to F. o. melonis.