| Twenty-two strains of Verticillium dahliae assigned to 15 of 16 vegetative compatibility groups (VCGs) by pairings of microsclerotial color mutants (Phytopathology 73:1305-1308), were reassessed by pairings of complementary, auxotrophic, nitrate nonutilizing (nit) mutants. Nit mutants were isolated by selecting for chlorate resistance. Two phenotypic classes of mutants (nitl and nitM) were identified on media with variable nitrogen sources. A nitl or nitM mutant was isolated from each of 22 strains and paired with a nitl and nitM mutants derived from tester strains representing 11 VCGs. Strains were considered vegetatively compatible when heterokaryons were produced between respective nit mutants as evidenced by dense prototrophic growth at the zone of mycelial contact. Results revealed that most strains judged incompatible by using color mutants were compatible when nit mutants were employed. Almost all strains judged compatible using color mutants were also compatible using nit mutants.;Four VCGs were detected among the 22 strains. VCGs 1, 2, and 4 were highly, but not absolutely incompatible with one another. Among VCGs 2 and 4, subdivisions were established (2A, 2B, 4A, 4B). Most strains within a subdivision complemented strongly with respective tester strains but only weakly with the tester strain of the other subdivision within the same VCG. Strains in VCG 3 were highly incompatible with strains in VCGs 1, 2, and 4B but partially-compatible with strains in VCG 4A. Among 184 strains of V. dahliae tested from Ohio, two belonged to VCG 1, 27 to VCG 2A, 25 to VCG 2B, 33 to VCG 4A and 90 to VCG 4B. Among 48 strains tested from diverse potato-producing areas of the U.S.A., 36 were assigned to VCG 4A, and 5 to VCG 4B. Greenhouse pathogenicity tests with 132 Ohio strains of V. dahliae demonstrated that all strains were virulent to potato cv. Superior. In addition, strains assigned to VCG 4A were significantly (P ;Vegetative compatibility was also tested among 60 strains from several hosts and geographical locations. Eleven were assigned to VCG 1, 30 to VCG 2, four to VCG 3 and ten to VCG 4. Strains isolated from the same host tended to belong to the same VCG(s). All 12 strains isolated from tomato and identified as race 1 were assigned to VCG 2 while four from race 2 were assigned to VCG 4. |
