GENETIC-VARIATION FOR FIELD-RESISTANCE TO VERTICILLIUM-DAHLIAE EVALUATED USING GENOTYPES AND SEGREGATING PROGENIES OF CALIFORNIA STRAWBERRIES

Resistance to wilt caused by Verticillium dahliae Kreb. was evaluated for 41 strawberry genotypes from the Univ. of California breeding prog ram and 1000 offspring from crosses among 23 of these genotypes, Runne r plants from these genotypes and seedlings were inoculated with a con idial Suspension containing a mixture of five isolates of V. dahliae f rom strawberry. Symptoms were scored as the number of dead or seriousl y stunted plants per plot, or based on a subjective phenotypic resista nce score assigned to each plot on five dates during the spring after planting. Most of the California germplasm is highly susceptible to V. dahliae with an average resistance score of 2.1 (+/-0.10) and 84.1% ( +/-2.1) plants stunted or dead compared with a score of 3.2 (+/-0.24) and 57.4% (+/-4.9) of plants stunted or dead for a control set of six non-California genotypes identified previously as resistant. However, a broad range of intermediate resistance was detected, and 4 of the 41 California genotypes evaluated had resistance scores superior to the mean score for the non-California resistant checks. Plot-mean heritabi lities for resistance and stunting scores estimated using genotypic, f ull-sib family, and offspring-patent analyses ranged from 0.44 to 0.88 . Comparison of different estimates of variance components suggests th at half or more of the genotypic variance for resistance traits detect ed is due to the additive effects of genes. There appears to be suffic ient variation within the California population to proceed with an eff ective selection program, despite the absence of directional selection for resistance during the past 3 decades. However, developing cultiva rs with adequate resistance will ultimately depend on the recovery of transgressive segregants from superior parents, as even the most resis tant genotypes from all sources showed some disease symptoms.