Molecular Variation Among Isolates of Verticillium dahliae and Polymerase Chain Reaction-Based Differentiation of Races

Verticillium dahliae is a soilborne fungal pathogen that causes vascular wilt in a variety of economically important crops worldwide. There are two races of V dahliae that infect tomato and lettuce. Although race-1-specific resistance has been identified in both tomato and lettuce, no resistant sources are available for race 2. Molecular analyses were employed to characterize the genetic variability and race structure of 101 isolates of V dahliae from a variety of hosts, mainly from central and coastal California, and 10 isolates exotic to this area. Analyses of the 16 simple sequence repeat (SSR) markers illustrated that tomato subpopulations from central California were distinct relative to the marigold subpopulations. In contrast, cotton and olive isolates showed admixture with tomato isolates. Analyses of both the ribosomal DNA intergenic spacer regions and SSR markers revealed high genetic variability among isolates but were unable to delineate races of V dahliae. However, a polymerase chain reaction (PCR) assay was applied to amplify a race-1-specific amplicon from the isolates in many hosts from different geographic areas, and was coupled with virulence assays for validation of the data. Results of the PCR assay showed 100% concordance with the virulence assay to differentiate race 1 from race 2 of 48 isolates from tomato. The results indicate that the PCR assay can be applied to differentiate the two races to support our related aim of breeding host resistance, and further reveal insights into the distribution of races in tomato and lettuce cropping systems in California.