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Douglas V. Shaw and Thomas R. Gordon

Strawberry (Fragaria ×ananassa Duch.) genotypes retained for resistance to Verticillium wilt (Verticillium dahliae Kleb.) after two cycles of a two-stage (TS) selection procedure consisting of full-sib family selection followed by within-family selection of individuals, and genotypes retained for resistance using genotypic mass (GM) selection were crossed to a common set of moderately susceptible genotypes. The relative resistance of the seedlings from these progenies was compared using a resistance score and the percentage of stunted plants. Although the two sets of resistant parents had performed similarly in genotypic comparisons, those genotypes selected using the TS procedure yielded test cross offspring with significantly higher resistance scores (X̄ = 3.84 ± 0.09 vs. X̄ = 3.46 ± 0.09, t = 3.11**) and significantly lower rates of plant stunting (X̄ = 38.1% ± 3.1 vs. X̄ = 50.2% ± 2.9, t = 2.87**) than the parents chosen using GM selection. Further resolution using analysis of variance and general combining ability (GCA) estimates showed that these between-set differences resulted from higher resistance breeding values for parents selected using the TS procedure. The five genotypes with largest GCA for resistance score and four of the five genotypes with minimum GCA for percentage stunting were obtained by TS selection.

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Thomas R. Gordon, Douglas V. Shaw, and Kirk D. Larson

Previous studies have demonstrated significant genetic variation for susceptibility to verticillium wilt, caused by Verticillium dahliae, among strawberry (Fragaria ×ananassa Duch.) genotypes adapted to California growing conditions. These evaluations have been conducted using a conidial root-dip inoculation procedure; valid application of this method in a breeding program assumes the reaction of inoculated plants will be predictive of their response to infection by more natural means. To test this expectation, we evaluated the responses of plants representing eight strawberry genotypes that were either root-dip inoculated prior to being transplanted into a fruit production field or were transplanted into soil artificially infested with pathogen propagules (microsclerotia). Both inoculation methods revealed significant variation among genotypes in all 3 years that tests were conducted and the absence of significant genotype × treatment interactions demonstrate similar rankings of genotypes with both methods. However, based on statistical repeatability, the root-dip inoculation method was more consistent over time (R = 0.759) than the soil inoculation method (R = 0.510).

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Douglas V. Shaw, Thomas R. Gordon, and Kirk D. Larson

Strawberry runner plants from the cultivar `Selva' (Fragaria ×ananassa Duch.) were produced using three nursery treatments in each of three years: propagation in soil fumigated with a mixture of 2 methyl bromide: 1 chloropicrin (w/w) at 392 kg·ha-1, propagation in fumigated soil but using planting stock inoculated prior to nursery establishment with a conidial suspension of Verticillium dahliae (106 conidia/mL), and propagation in nonfumigated soil naturally infested with V. dahliae. Runner plants were harvested and stored at 1 °C for 6, 18, or 34 days prior to establishment in fruit production trials. No significant differences were found between runner plants grown in naturally infested soil and runner plants obtained from artificially inoculated mother plants for V. dahliae infection rates detected by petiole isolation immediately prior to transplanting, the percentage of plants visibly stunted due to disease during the following production season, and seasonal yield compared with corresponding noninfected controls. Cold storage of runner plants for 18 or 34 days, produced using either natural or artificial inoculation systems, reduced the initial percentage of infected plants by 42% to 61% and the percentage of stunted plants during the following fruit production season by 43% to 57%, compared with plants from corresponding nursery treatments given only 6 days post-nursery cold storage. Yields for inoculated plants with 6 days cold storage were 16% to 20% less than those for uninoculated controls, whereas yields for inoculated plants with 18 or 34 days of storage were 3% to 9% less than the respective controls. Most of the cold storage effects on initial infection rate, stunting, and yield were realized at the 18 days of storage treatment. A reduction in the fraction of V. dahliae infected plants due to cold storage, suggests either a direct effect of cold storage on the disease organism or stimulation of secondary resistance mechanisms in the plant. Chemical name used: trichloronitromethane (chloropicrin).

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Thomas R. Gordon, Sharon C. Kirkpatrick, Douglas V. Shaw, and Kirk D. Larson

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.

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Thomas R. Gordon, Dorothy Okamoto, Andrew J. Storer, and David L. Wood

Pitch canker, caused by Fusarium subglutinans f. sp. pini, causes branch dieback and stem cankers in many species of pine. Monterey pine (Pinus radiata D. Don), one of the most widely planted pines in the world, is extremely susceptible to pitch canker. Four other pine species, which might serve as alternatives to Monterey pine in landscape settings, were found to be relatively resistant, based on the size of lesions resulting from branch inoculations under greenhouse conditions. Of these species, Japanese black pine (P. thunbergiana Franco) was the most resistant, followed by Canary Island pine (P. canariensis Sweet ex K. Spreng), Italian stone pine (P. pinea L.), and Aleppo pine (P. halepensis Mill.). Consistent with these findings, a field survey conducted in Alameda County, Calif., revealed Monterey pine to have the highest incidence of infection, with significantly lower levels in Aleppo, Canary Island, and Italian stone pines. Japanese black pine was not observed in the survey area.

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Thomas R. Gordon, Dorothy Okamoto, Andrew J. Storer, and David L. Wood

Pitch canker, caused by Fusarium subglutinans f. sp. pini, causes branch die-back and stem cankers in many species of pine. Monterey pine (Pinus radiata D. Don), one of the most widely planted pines in the world, is extremely susceptible to pitch canker. Four other pine species, which might serve as alternatives to Monterey pine in landscape settings, were found to be relatively resistant, based on the size of lesions resulting from branch inoculations under greenhouse conditions. Of these species, Japanese black pine (P. thunbergiana Franco) was the most resistant, followed by Canary Island pine (P. canariensis Sweet ex K. Spreng), Italian stone pine (P. pinea L.), and Aleppo pine (P. halepensis Mill.). Consistent with these findings, a field survey conducted in Alameda County, Calif., revealed Monterey pine to have the highest incidence of infection, with significantly lower levels in Aleppo, Canary Island, and Italian stone pines. Japanese black pine was not observed in the survey area.

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Douglas V. Shaw, Thomas R. Gordon, Kirk D. Larson, and Sharon C. Kirkpatrick

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.