Strawberry genotypes were retained from biparental progenies previously identified as either relatively susceptible or resistant to wilt caused by Verticillium dahliae based on a phenotypic resistance score. Runner plants from these selected genotypes were inoculated with a conidial suspension containing a mixture of five isolates obtained from symptomatic strawberry plants at 106 conidia/mL, then scored for disease symptoms. Genotypes from resistant progenies had significantly higher phenotypic resistance scores (1 = severe symptoms, 5 = no detectable symptoms) than those from susceptible progenies (4.15 vs. 2.23), and there also was a correlated selection response for the percent plants severely stunted or dead (26.4 and 69.1 for resistant and susceptible selections, respectively). Comparisons of the resistance scores for selected groups with those for the original parents (2.76) indicated that selection had changed relative resistance significantly in both directions and that realized response had been 24% to 43% larger than predicted for selection in both directions. Although several of the selections from resistant progenies were highly susceptible, five of the 21 resistant selections had resistance scores outside the range of the original parents, representing possible transgressive segregants. This comparison is limited by the precision with which individual resistance scores are estimated and by the scope of the disease symptoms in this trial. Detecting genotypes with sufficient resistance ultimately will depend on development of screening methods with greater sensitivity than those used here.
Resistance to wilt caused by Verticillium dahliae Kreb. was evaluated for 41 strawberry genotypes from the Univ. of California breeding program and 1000 offspring from crosses among 23 of these genotypes. Runner plants from these genotypes and seedlings were inoculated with a conidial suspension containing a mixture of five isolates of V. dahliae from strawberry. Symptoms were scored as the number of dead or seriously stunted plants per plot, or based on a subjective phenotypic resistance 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 heritabilities for resistance and stunting scores estimated using genotypic, full-sib family, and offspring-parent analyses ranged from 0.44 to 0.88. Comparison of different estimates of variance components suggests that half or more of the genotypic variance for resistance traits detected is due to the additive effects of genes. There appears to be sufficient variation within the California population to proceed with an effective selection program, despite the absence of directional selection for resistance during the past 3 decades. However, developing cultivars with adequate resistance will ultimately depend on the recovery of transgressive segregants from superior parents, as even the most resistant genotypes from all sources showed some disease symptoms.