ASSESSMENT OF VEGETATIVE COMPATIBILITY OF VERTICILLIUM-DAHLIAE TESTER STRAINS AND ISOLATES FROM CALIFORNIA POTATOES

Many strains of Verticillium dahliae previously considered vegetatively incompatible based on microsclerotial color mutants were found to be compatible when reassessed using nitrate-nonutilizing (nit) mutants. Sixteen vegetative compatibility groups (VCGs) were found previously when strains of V. dahliae were assayed using microsclerotial color mutants. When 26 strains representing 16 VCGs were reassessed using nit mutants, only four VCGs were identified. Three strains were assigned to VCG 1, 13 to VCG 2, seven to VCG 4, and one to VCG 5. The two remaining strains could not be assigned to any of the identified VCGs. Within VCG 2 and 4, subgroups were evident when percentages of similarities were compared. Strains were assigned to VCGs based only on strong compatibility reactions, because weak and incompatible reactions were unreliable. Isolates from potatoes growing in the Bakersfield region of California were only compatible with tester strains from VCG 1. Twenty-six of 30 isolates from potatoes growing in the Tulelake region of California were only compatible with strains from VCG 4. The remaining four were not strongly compatible with any tester strains, and thus may represent new VCGs. Two strains from VCG 4, TA and 277, were found to be significantly (P = 0.05) more virulent than strains PU, MC, 207, and PCW on the potato cultivar Netted Gem.