VEGETATIVE COMPATIBILITY AND VIRULENCE OF STRAINS OF VERTICILLIUM-DAHLIAE FROM SOIL AND POTATO PLANTS

One-hundred and eighty-seven wild-type strains of Verticillium dahliae were isolated from potato plants and soil from 22 potato fields in Ohio. Strains were assigned to vegetative compatibility groups (VCGs) based on pairings of complementary nitrate-nonutilizing (nit) mutants induced on a chlorate-containing medium. Of these strains, two were assigned to VCG 1, 53 to VCG 2, and 128 to VCG 4. The remaining four strains could not be tested for vegetative compatibility because of their inability to yield nit mutants. Forty-seven additional strains of V. dahliae isolated from potato plants were obtained from nine U.S. states and tested for vegetative compatibility. Of these strains, two were assigned to VCG 2 and 45 to VCG 4. VCG 4 of both isolate collections was further subdivided into VCG 4A and VCG 4B to accommodate differential vegetative compatibility reactions of strains from each subgroup when paired against tester strains of VCG 3. Most strains assigned to VCG 4A were weakly vegetatively compatible with tester strains of VCG 3. In contrast, all strains in VCG 4B were vegetatively incompatible with tester strains of VCG 3. Greenhouse pathogenicity tests with 129 strains of V. dahliae from Ohio assigned to VCGs 2, 4A, and 4B were conducted by root-dipping potato sprouts (cultivar Superior) in a suspension of 10(5) conidia per milliliter of each isolate. Areas under senescence progress curves (AUSPCs) were computed for each strain. Contrasts of mean AUSPCs for each VCG revealed that most strains in VCG 4A were significantly (P = 0.01) more virulent than those in VCGs 2 and 4B, which is evidence for recognition of two distinct pathotypes.