Strawberry breeding improves genetic resistance to Verticillium wilt

Since 1994, more than 480 genotypes from the UC strawberry breeding program have been screened for resistance to verticillium dahliae Kleb., an important soil pathogen of strawberry. Genotypes for parents of subsequent generations have been chosen using a multiple-trait strategy that incorporates their verticillium re- sistance rating. This selection strategy has increased resistance scores for the parents by 60%, and increased the percentage of moderately resis- tant genotypes from 35.0% in the original germplasm to 78.5% in those used as parents for the most recent crosses. Selection has reduced genetic variation for the resistance score, and genotypic coefficients of variation (GCV) decreased in the breeding pop- ulation from 34.4% to 11.6% from 1994 to 2008. Inspection of genotypic scores suggests that the GCV change pattern may not be due to a scarcity of variation, but rather to limitations in the detection test. Our results sug- gest the need for broader testing of the more-resistant types identified in naturally infested soils and improved understanding of resistance mecha- nisms. Ultimately, this work seeks to provide a verticillium-resistant culti- var to growers if access to effective soil fumigants becomes more limited.