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  • Author or Editor: Douglas V. Shaw x
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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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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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Controlled pollinations were made using 20 elite selections from the University of California, Davis, Prunus domestica (european plum) breeding program as parents. These parents were used to generate 11 self-pollinated progenies with an inbreeding coefficient (F) of 0.5, 10 full-sibling progenies (F = 0.25), and 11 progenies from among nonrelated parents (F = 0). Seven additional progenies were chosen as a random-mating control set within the parental group; progenies in the control set had accumulated a range of current inbreeding coefficients (average F = 0.23) over two to five generations with intervening cycles of selection. Survival percentages were 85, 82, and 74 for the full-sib progeny, control set progeny, and selfed progeny, respectively, relative to nonrelated progeny. Two months after germination the percent decrease in the growth trait means for the selfed progeny compared to the nonrelated progeny ranged from 14% to 30% whereas growth trait means for full-sib progeny decreased from 1% to 9% compared to nonrelated progeny. The percent decrease for growth trait means of the selfed progeny after completing one season of growth in the field (10 months) was similar to that observed after 2 months, ranging from 14% to 28% compared to nonrelated progeny, whereas the decrease in full-sib progeny trait means was somewhat greater, ranging from 6% to 20%. Regression analysis of all growth traits on current-generation rates of inbreeding indicated a significant negative linear relationship (P = 0.0011 to 0.0232). No significant relationships were found between accumulated Fs and growth trait means of the control set progenies and the nonrelated progenies after 2 months in the greenhouse or one season growing in the field, suggesting that selection between breeding cycles decreased inbreeding depression.

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Eighteen strawberry genotypes from the University of California's breeding population were evaluated over two years for yield and fruit size with complete, partial, and no control of natural infestation by Tetranychus urticae Koch. The numbers of mites per leaf accumulated for the entire season or counted at peak infestation, and the number of mite-days accumulated for the season for partial control treatments were 31.7% to 44.0% of corresponding values realized for uncontrolled infestation, and values differed significantly between treatments for all three variables. Yields for the no-control and partial-control treatments averaged 81.6% and 85.0% of the yields obtained with complete spidermite suppression for the 2 trial years; fruit sizes were 95.1% and 92.0% for corresponding comparisons. Yield and fruit size differed significantly between the complete-control treatment and any level of infestation, but statistically significant differences between partial and complete mite control treatments were detected only for fruit size in a single year. Analysis of variance demonstrated significant or highly significant variation due to control level, genotype, and their interactions for both yield and fruit size, but resolution of variance components demonstrated that genetic × treatment interactions explained just 0% to 8% of the phenotypic variance for yield and fruit size in a 2-year evaluation. Genotypic variances, those reflecting genetic effects that were stable across treatments, were at least 9.3 times as large as interaction variances for these traits. There appears to be no evidence for partial resistance that might be expressed at intermediate levels of spidermite infestation.

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Nine selected strawberry genotypes (Fragaria ×ananassa Duch.) from the University of California, Davis, strawberry improvement program were intercrossed and their seedling offspring evaluated for five production traits. Plants were evaluated in annual hill culture, with and without preplant soil fumigation using a mixture of 67 methyl bromide: 33 chloropicrin (wt/wt, 392 kg/ha). Plant mortality was <1% for seedlings grown in either soil environment, indicating that the main effects of fumigation treatment in this experiment were due to the consequences of sublethal soil organisms. Plants grown in nonfumigated soils measured from 74% to 77% of the diameter of those grown in fumigated soils and yielded 59% as much fruit. Significant cross × fumigation interactions were not detected for fruit yield, fruit size, and weighted fruit appearance. Moreover, genetic correlations for these three traits calculated by comparing seedling performance in fumigated and nonfumigated soil environments were at or near unity, suggesting that the same genes condition genetic variability for these traits in both soil environments. Together, these findings demonstrate that strawberry fruit yield and vigor are increased substantially by fumigation, even in the absence of an identifiable major pathogen problem. Further, there may be little promise for developing cultivars with genetic adaptation specific to the sublethal effects of nonfumigated soils, as selection in either soil fumigation environment is likely to affect the same sets of genes.

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Abstract

Genetic variation in vigor, phenology, and branching in juvenile seedlings of Juglans californica and J. hindsii was investigated. Significant differences between species were detected in traits that reflect vigor (height, diameter, volume), in phenological traits (dates of leafing out and leaf drop), and in branching. J. californica was generally more vigorous, more branched, leafed out earlier, and dropped leaves later than J. hindsii. When species were analyzed separately, seed source was a highly significant source of variation for phenological traits and branching in J. hindsii. Upper estimates of heritability for phenological traits ranged from 0.47 to 0.88. The results of this study suggest that J. hindsii may have had a history of differentiation and adaptation to latitude, elevation, or other climatic characters. Conservation of germplasm resources in J. hindsii will be essential to maintain the purity of the species and to provide resources for studying the species and breeding rootstock for J. regia, the Persian walnut.

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Abstract

Strawberry (Fragaria × ananassa Duch.) seedlings from biparental crosses were planted in two trials at Watsonville, Calif. Sept. 1985 and 1986. The 1985 trial was scored on three dates and the 1986 trial on four dates for leaf spot (Ramularia tulasnei) infection. Individual seedlings were assigned a value of 0 to 5 on each date, with 0 indicating no infection and 5 indicating very severe infection. Average infection scores were lowest for the earliest scoring dates in both years (0.93 and 0.38) and maximum at late scoring dates (2.83 and 1.49). Narrow-sense heritabilities for infection scores estimated using partial diallel analyses were 0.07 and 0.03 for the earliest scoring dates in the two trials and were 0.24 and 0.28 for the latest dates. Early season estimates of broad-sense heritabilities were calculated as 0.13 and 0.03, whereas estimates based on later scorings ranged from 0.32 to 0.52. The low heritabilities for early scoring dates result from incomplete and uneven infection, indicating that selection under field conditions will be most effective under conditions of very high infection. The moderate heritabilities and predicted gains estimated for high-infection conditions suggest considerable opportunity for improvement of leaf spot resistance by recurrent breeding and selection using available quantitative genetic variation.

Open Access

Abstract

Genetic variation for percent soluble solids, percent titratable acidity, and fruit firmness was analyzed for representative progenies from the California strawberry breeding population. Fruit samples were harvested from individual seedlings from 28 biparental crosses, organized in two factorial mating sets. Individual narrow-sense heritabilities were estimated as 0.07, 0.48, and 0.38 for solids, acids, and firmness, respectively. Broad-sense heritabilities were estimated as 0.35, 0.78, and 0.38 for the same traits, suggesting the presence of dominance variance for soluble solids and acidity, but not for firmness. Opportunities for genetic manipulation of traits with different additive-dominance genetic variation profiles are discussed.

Open Access

Eight strawberry cultivars or advanced selections from the Univ. of California, Davis, breeding program were screened for polymorphisms using the polymerase chain reaction (PCR) and 43 random 10-base DNA primers. Over 60% of the primers screened resulted in replicable polymorphic banding patterns (amplification profiles), and a subset of ten primers that exhibited high levels of amplification profile polymorphism was used to identify each of the eight genotypes uniquely. There was also a significant product-moment correlation (r = 0.64, P < 0.01) between number of shared amplification profile phenotypes and pairwise coefficient of coancestry. This technology shows high promise as a means of verifying the identity of cultivars and developing a genetic map of the octoploid cultivated strawberry.

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One hundred-eighty six strawberry genotypes from the Univ. of California strawberry (Fragaria ×ananassa Duch.) breeding program were evaluated for resistance to Phytophthora cactorum Schroet. in trials conducted over 6 years; 60 of these genotypes were tested in 2 years or more. Mother plants of each genotype were grown in a propagation nursery beginning in June, and runner plants were set into soil infested with inoculum from a mix of four P. cactorum isolates in August or September of the same year. Runner plants of each genotype were harvested from the inoculated nursery, transferred to a fruiting field location, and evaluated for disease symptoms during the winter and spring following inoculation using a disease severity score. Significant variation for the disease severity score was detected due to years, genotypes, and their interaction. Differences among genotypes were responsible for 60.6% of the phenotypic variance, whereas years and year × genotype interactions contributed relatively little to this variance, 8.2% and 9.3%, respectively. A separate analysis conducted using a balanced subset of six cultivars that were present in all trial years detected variance components due to years and year × genotype interaction slightly smaller than those estimated for the complete trial, 5.0% and 3.9%, respectively. These results highlight the utility of the screening system and suggest that stable resistance to P. cactorum is obtainable in California strawberry breeding populations and production systems.

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