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Forty-eight strawberry (Fragaria ×ananassa Duchesne) or (Fragaria L. sp.) genotypes from the University of California advanced-cycle breeding population were evaluated over 7 years for susceptibility to and tolerance of infestation by two-spotted spider mite (Tetranychus urticae Koch). In pairwise tests, 23 photoperiodically short-day genotypes were compared with the short-day cultivar Chandler, and 23 day-neutral genotypes to the day-neutral cultivar Selva. Feeding by T. urticae resulted in substantial yield reductions regardless of genotype. Yield reduction from feeding by T. urticae averaged 29.9% for short-day and 23% for day-neutral genotypes. Calculation of variance components for day-neutral genotypes determined that none of the variation in yield was explained by the interaction of genotype and T. urticae infestation, whereas 24.4% of the variation in yield for short-day genotypes was explained by the interaction of genotype and T. urticae infestation. Under current strawberry production practices in California there appears to be little potential for breeding direct resistance to T. urticae for day-neutral genotypes. However, some gains in breeding direct T. urticae resistance may be achieved within short-day genotypes. Phenotypic path-coefficient analysis for direct and indirect effects, and simple correlation coefficients of T. urticae feeding determined there were substantial differences between short-day and day-neutral genotypes in their yield responses to T. urticae feeding. For short-day genotypes, the greatest direct effect on yield resulted from T. urticae feeding in June. For day-neutral genotypes, the greatest direct effects resulted from T. urticae feeding in April and May and were probably due to the day-neutral genotype's more complex flowering responses. Several high-yielding cultivars have been developed and released over the 7 years of this study. It appears the new cultivars that were bred for current cultural practices exhibited a consistent plant response for greater yield both in the presence and absence of T. urticae feeding.

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.

Strawberry cultivars Selva and Camarosa (Fragaria ×ananassa Duch.) were grown at a high elevation nursery in soil that was either naturally infested with Verticillium dahliae or was rendered pathogen-free through preplant fumigation with 2 methyl bromide: 1 chloropicrin (wt/wt) at 392 kg·ha^-1. Plants grown in fumigated soil were inoculated with a conidial suspension of V. dahliae, prior to establishment. Just prior to harvest, plants were rated for disease based on symptoms of Verticillium wilt. At the same time, petiole samples were taken from mother plants and each of three generations of runner plants, along with the stolons subtending each of the sampled runner plants. Petioles and stolons were cultured to assay for the presence of V. dahliae, and scored as either infected or not infected. The experiment was conducted in each of two successive years, and the following conclusions were supported by results obtained in both years. First, symptoms of Verticillium wilt on mother plants of both cultivars were highly correlated with recovery of V. dahliae from petioles, but runner plants were consistently free of symptoms even though they were often infected. Second, runner plants sustained lower infection frequencies than mother plants, with the differences being significant in most cases. Lastly, infection of runner plants was due, at least in part, to transfer of inoculum from infected mother plants; in some cases this appears to have been the exclusive mode of infection.

Mother plants from strawberry (Fragaria ×ananassa Duch.) genotypes susceptible to and resistant to verticillium (Verticillium dahliae Kleb.) were inoculated with this pathogen in a high elevation nursery. The infection rate for mother plants was 77.3% ± 7.5% and 80.7% ± 5.4% for resistant and susceptible genotypes, respectively. Conversely, the percentage of runner plants identified as infected by postharvest petiole assay that were produced by these inoculated mother plants differed significantly (P < 0.01) between sets of genotypes, 25.1% ± 3.7% and 59.8% ± 5.3% for resistant and susceptible genotypes, respectively. The percentage of runner plants from inoculated mothers that eventually collapsed in the fruit production field was larger than the percentage identified as infected by petiole assay for susceptible genotypes (68.9% ± 4.1%), and substantially less than the pre-plant infected fraction for resistant genotypes (3.5% ± 1.4%). Yield for runner plants from inoculated mothers was reduced by 73% to 75% for susceptible genotypes, and by 7% to 15% for resistant genotypes. The percentage of runner plants identified as infected in the nursery was correlated with the percentage of plants that collapsed in the fruiting field (r = 0.91, P < 0.01) and with yield in infested plots (r = -0.79, P < 0.01). Most of the effect of this disease was expressed as plant collapse, but the presence of yield reductions larger than the rate of plant collapse demonstrated substantial sub-lethal effects as well.

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.