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.