Fusarium oxysporum is an ascomycetous fungus that is well-known as a soilborne 
plant pathogen. In addition, a large population of nonpathogenic F. oxysporum 
(NPF) inhabits various environmental niches, including the phytosphere. To 
obtain an insight into the origin of plant pathogenic F. oxysporum, we focused 
on the tomato (Solanum lycopersicum) and its pathogenic F. oxysporum f. sp. 
lycopersici (FOL). We collected F. oxysporum from wild and transition Solanum 
spp. and modern cultivars of tomato in Chile, Ecuador, Peru, Mexico, 
Afghanistan, Italy, and Japan, evaluated the fungal isolates for pathogenicity, 
VCG, mating type, and distribution of SIX genes related to the pathogenicity of 
FOL, and constructed phylogenies based on ribosomal DNA intergenic spacer 
sequences. All F. oxysporum isolates sampled were genetically more diverse than 
FOL. They were not pathogenic to the tomato and did not carry SIX genes. Certain 
NPF isolates including those from wild Solanum spp. in Peru were grouped in FOL 
clades, whereas most of the NPF isolates were not. Our results suggested that 
the population of NPF isolates in FOL clades gave rise to FOL by gaining 
pathogenicity.