VCG diversity and virulence of Verticillium dahliae from commercially available cotton seed lots in Turkey

Samples of seeds collected from commercially available cotton seed lots in Turkey were assayed for Verticillium dahliae. V. dahliae was successfully isolated from 67 of 104 seed lots tested, a successful isolation rate of nearly 65 %. Vegetative compatibility of the isolates was assessed through complementation tests using nitrate non-utilizing mutants. Of the 188 isolates obtained, 105 were classified as VCG1A, 17 as VCG2A, 64 as VCG2B and two as VCG4B. All 50 of the isolates tested in the greenhouse on cotton cv. DP 15-21 and Acala SJ-1 were pathogenic on both cultivars. As a group, AUDPC values were significantly higher (P < 0.05) for VCG1A than for VCG2 and VCG4B isolates. These data suggest that: (1) commercial cotton seed lots are commonly infected with V. dahliae and thus may serve as primary sources of the pathogen; (2) cotton isolates of V. dahliae belong to VCG1A, 2A, 2B and 4B and these strains are widely distributed via seed lots; and (3) VCG1 and VCG2 are distinct pathotypes of V. dahliae that vary in their virulence to cotton.