Reassessment of vegetative compatibility relationships among strains of Verticillium dahliae using nitrate-nonutilizing mutants

Twenty-two strains of Verticillium dahliae, originally assigned to 15 vegetative compatibility groups (VCGS) using microsclerotial color mutants, were tested for vegetative compatibility using complementary, auxotrophic nitrate-nonutilizing (nit) mutants. Nit mutants were generated from wild-type strains of V. dahliae by selecting chlorate-resistant sectors on cornmeal agar with dextrose amended with potassium chlorate (15-25 g/l). Complementation tests between nit mutants derived from these strains led to the identification of only four distinct VCGs. These results demonstrated that many strains considered to be incompatible when microsclerotial color mutants were used to test vegetative compatibility were compatible when nit mutants were employed. With the exception of strain PU, which was heterokaryon self-incompatible, all strains that were considered vegetatively compatible with the color mutant technique were also compatible when nit mutants were employed. Complementation tests, between nit mutants derived from 21 additional strains of V. dahliae from Ohio and tester strains representing the four VCGs, confirmed the existence of these four groups. Distribution of tested strains among the four VCGs were: three in VCG 1, 21 in VCG 2, two in VCG 3, and 15 in VCG 4. All strains within a VCG were strongly compatible with at least one of the selected tester strains but were not always completely incompatible with strains of other VCGs