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Performance characteristics for eighteen strawberry cultivars (Fragaria ×ananassa), nine from California and nine from other North American sources, were evaluated in annual hill culture, with and without preplant soil fumigation (2 methyl bromide : 1 chloropicrin, 392 kg·ha^-1). Plants grown in nonfumigated soil yielded 57% and 46% of the fruit produced by plants on adjacent fumigated soil for cultivars from California and other North American origins, respectively. Plants in nonfumigated soils also developed fruit with lower berry weight (94% and 95% of fumigated trials) and smaller spring plant diameter (83% and 76%) for California and other sources, respectively. Trait values for exotic cultivars ranged from 39% to 80% of those for California cultivars, and the variance component due to germplasm sources explained 41% to 81% of the phenotypic variance of random effects in the experiment. Conversely, significant germplasm source × fumigation interactions were not detected for any of the growth or performance traits evaluated, and the proportion of variance attributable to these interactions was at most 2% of that due to germplasm source. These results demonstrate that strawberry growth and productivity for California and other North American germplasm sources are increased similarly by fumigation. Despite differing selection history, germplasm developed outside of California contains no obvious genetic diversity useful for developing cultivars specifically adapted to the sublethal effects of organisms in nonfumigated soils.

Bare-rooted `Camarosa' strawberry runner plants were established in a fruit production field on 1 Nov. 1993 using annual hill culture and two preplant soil fumigation treatments: 1) a mixture of 2 methyl bromide: 1 chloropicrin (wt: wt, 392 kg·ha^-1) injected into the soil before forming raised planting beds (MBC); or 2) nonfumigation (NF). At about 33-day intervals between mid-January and the end of May, 20 plants were destructively sampled from each treatment to determine leaf dry mass (LDM), crown dry mass (CDM), root dry mass (RDM), and shoot: root dry mass (SRDM) ratios. Plant mortality was <0.2% throughout the study and did not differ with soil treatment. Regardless of sampling date, LDM, CDM, and RDM were greater for MBC plants than for NF plants, although treatment differences were not always significant. During the first 143 days, NF plants allocated a greater proportion of dry matter to roots than to shoots compared to MBC plants, indicating that roots are a stronger sink for photoassimilate in nonfumigated than in fumigated soils. However, there was no difference between treatments in SRDM by the end of the study. Fruit yield and a 10-fruit weight were determined at weekly intervals from mid-January until 23 May 1994. Yield and mean fruit weight of NF plants were 72% and 90%, respectively, of that of MBC plants. For both treatments, about one-half of total fruit production occurred between 144 and 174 days after planting (late March to late April). During that same period, rates of dry matter accumulation in leaf, crown, and root tissues decreased for plants in both treatments, but greatest reductions occurred in NF plants. Chemical name used: trichloronitromethane (chloropicrin).

Seedling offspring of crosses among 10 selected strawberry genotypes (Fragari ×ananassa Duch.) from the University of California strawberry improvement program were established in annual hill culture. Soil treatments consisted of 1) preplant fumigation using a mixture of methyl bromide and chloropicrin or 2) no fumigation. Root systems of individual plants were sampled with a soil probe in January, April, and July 1994 to determine root mass (RM), secondary root mass (SRM), and a subjective root appearance score (RAS). For each trait, genetic analyses of partial diallels were performed to quantify sources of genetic, environmental, and interaction variance. Root trait values differed significantly between soil treatments only for the April sampling date, with all trait values greater in fumigated soils than in nonfumigated soils. For RM and SRM, variance due to general combining ability (GCA) was significant in April and July. Narrow-sense heritabilities (h² ) for RM increased between January (0.14) and July (0.40); SRM showed a similar trend with a higher h² on each sampling date. GCA variances were nonsignificant for RAS, however, significant fumigation × GCA interaction variance was detected for RAS in January. Specific combining ability (SCA) variances were nonsignificant for all traits. To further quantify the extent of interactions, correlations (r_g ) between genotypic expressions in fumigated soils and nonfumigated soils were calculated for each root trait. These r_g values were at or near unity (> 0.85) for RM and SRM on all sampling dates, implying that genetic variability for these traits is conditioned by genes with identical effects within each soil environment. Conversely, r_g between soil environments was 0.52, 0.62, and -0.18, for January, April, and July RAS, respectively. These findings suggest that genetic variability exists within this germplasm base for strawberry root mass characteristics. Genetic variation also exists for January root appearance score, but it is not conditioned identically across fumigation treatments.

Genotypic and phenotypic relationships among root system and above-ground traits of strawberry (Fragaria ×ananassa Duch.) were evaluated for seedlings grown in annual hill culture, with soil treatments consisting of 1) preplant fumigation with methyl bromide and chloropicrin or 2) nonfumigation. Seedlings were from crosses among 10 genotypes within the University of California strawberry improvement program that had been selected previously for yield and other production traits. Root mass had positive genotypic correlations with plant diameter 5 months after planting in both fumigated (r = 0.58) and nonfumigated (r = 0.69) soils. Genotypic correlations between root mass and two production traits, yield and fruit size, were nonsignificant. However, plant diameter had positive genotypic correlations with yield (r = 0.36 to 0.51) and negative genotypic correlations with fruit size (r = -0.47 to -0.60). In general, root appearance scores were uncorrelated with production traits, but their genotypic correlations with vegetative traits were occasionally strong. Genotypic path coefficient analyses conducted separately for fumigated soils and nonfumigated soils both indicated that plant diameter had positive direct effects on yield that were twice the magnitude of that for any other trait. Root mass had a small negative direct effect on yield in each fumigation environment, while root appearance scores had small to moderate direct effects on yield that were more positive for samples obtained after fruiting (in April) versus before fruiting. Pleiotropic relationships appear to exist between root traits and plant diameter, but plant diameter is the best single predictor of genotypic variation for yield in both soil fumigation environments.

Four sets of selected strawberry (Fragaria ×ananassa Duch.) genotypes were generated from within a single breeding population to evaluate the correspondence between predicted and realized selection response for fresh fruit color traits. Genotypes were selected for extreme phenotypes, dark or light, of either internal or external color value (CIELAB L^*). Genotypic selection response was evaluated empirically by scoring fruit from the clonal derivatives of these selected genotypes, and response for breeding value was estimated by scoring the offspring of crosses performed among a subset of the genotypes within each selected set. Realized selection response was slightly larger than predicted for internal and external L^* when calculated for selected genotypes. Also, more than half of the selected genotypes had genotypic values for L^* outside the range of the original parents, providing evidence for transgressive segregation. Realized selection response for breeding value in exterior and interior color was slightly less than predicted. Compared in a different way, genotypic selection response for external color was significantly greater than selection response for breeding value, whereas genotypic and breeding value responses did not differ for internal color. These observations suggest the presence of some nonadditive genetic variance for external color but support the conclusion that the heritabilities predicted previously were reasonably accurate. Estimates of variance components within each of the offspring populations demonstrated that genetic variances were modified substantially by one generation of selection. Selection for dark fruit color reduced genetic variance to nonsignificant levels, with internal color more affected than external color. The total genetic variances within both of the offspring populations from parents selected for light color were changed little by one generation of selection, but substantial dominance variance was detected that had not been found in the original population. The rapid response to selection and large changes in the distribution of genetic variances may indicate the presence of a few genes with comparatively large effect in strawberry color expression. Additional divergent selection response can be expected, but primarily in the direction of light fruit color.

Performance characteristics for 12 strawberry genotypes (Fragaria ×ananassa Duch.) from the Univ. of California, Davis, strawberry improvement program were evaluated in annual hill culture, with and without preplant soil fumigation using a mixture of 67 methyl bromide:33 chloropicrin (trichloronitromethane) (wt/wt, 392 kg·ha^-1). Plants were established at two locations; one trial followed several cycles of strawberry plantation, whereas the other had not been cropped with strawberries for 20 years. Plant mortality was <3% and did not differ between soil treatments; thus, the main effects of fumigation treatment in these experiments were due to sublethal effects of soil organisms. Plants grown in nonfumigated soil produced 51% and 57% of the fruit yield of plants grown in fumigated soil for soils with and without a recent history of strawberry cultivation, respectively. Nonfumigated treatments also had reduced fruit weight and uniformly lower vegetative vigor during the early phases of plantation establishment. Significant genotype x fumigation interactions were not detected for any of the growth or performance traits at either location. Further, the proportion of variance attributable to interactions was at most 25% of that due to variation among genotypes, even for this highly selected population. Genotypic correlations for traits evaluated in different fumigation treatments ranged from 0.80 to 1.00; thus, selection in either soil environment is expected to affect largely the same sets of genes. These results demonstrate that strawberry productivity is substantially increased by fumigation, even in the absence of lethal pathogens or a discernible replant problem. More importantly, there appears to be little opportunity for developing cultivars specifically adapted to sublethal effects of nonfumigated soils.

Runner plants from 16 strawberry (Fragaria ×ananassa Duch.) cultivars were grown using annual Mediterranean production systems to test for differences in productivity, performance traits, and vegetative growth attributes. Genotypes were included from germplasm adapted to four geographic regions: California and northwestern, northeastern, and mid-Atlantic or southeastern United States. The California genotypes were divided further into day-neutral and June-bearing categories. With these treatments, California cultivars had significantly larger plants and grew more rapidly during the fall and winter, had larger fruit, and produced at least twice the quantity of fruit of cultivars from the other regions. Variance components due to region explained 64% and 26% of the phenotypic variance for early and total yield, respectively, whereas differences among cultivars within regions explained 12% and 7% of the variance for these traits. Cultivars from all regions had significantly larger plants and were more productive when treated with 3 weeks of artificial vernalization. However, region × vernalization effects were nonsignificant for all traits, a result suggesting that selection in Mediterranean environments has not adapted germplasm specifically for low vernalization conditions.

Yield and fruit size were determined for 49 strawberry (Fragari ×ananassa Duch.) genotypes during a 7 year period, in soils prepared with and without preplant soil fumigation using 2 methyl bromide: 1 chloropicrin (wt/wt). Strawberries were grown in alternate years, with the nonfumigated treatment representing the first, second, third, and fourth strawberry crop cycles initiated without soil fumigation. Highly significant (P < 0.01) effects of soil fumigation treatment were present for yield in a combined analysis over all years; fumigation increased yield by 41% over nonfumigated soils in the first nonfumigated cultivation cycle and by 68% to 74% for subsequent nonfumigated cycles. Fruit size was less affected by soil treatment but increases due to fumigation (2% to 18%) were significant (P < 0.05) in the third or fourth nonfumigated crop cycle. Genotypic variances were highly significant in the combined analysis, whereas geneti × fumigation interaction variances were significant only for fruit size and contributed <8% of the total phenotypic variance for either trait. Genetic correlations were r _g = 0.77 and 0.92, respectively, for yield and fruit size treated as independent traits across soil fumigation environments. There was no evidence for genes that confer specific adaptation to nonfumigated soils, or that these genes emerge as important contributors to the phenotypic variation as the soil environment deteriorates with repeated cultivation of strawberry in nonfumigated soil. Chemical names used: trichloronitromethane (chloropicrin).

To determine the best sampling strategies, components of variance were estimated for 10 color traits of strawberry (Fragaria × ananassa) fruit. Over three dates in one growing season, 2000+ fruit from 47 genotypes were observed. Within-fruit, among-fruit within genotypes, and harvest date variances were compared. Variances for harvest dates were non-significant or small (0-8% of the total variance). Genotype × date variances were highly significant but small (≤ 6% of the total) for all color traits except internal hue (14% of the total). For external color traits, the within-fruit variance was greater than the among-fruit variance (16-64% and 0-14% of the total, respectively). For internal color traits, the among-fruit variance was greater than the within-fruit variance (20-37% and 9-19% of the total, respectively). Obtaining two observations per fruit for multiple fruit on one harvest date is an efficient strategy for determining a genotype's fruit color. With two observations per fruit, 7 to 22 fruit are needed to estimate a genotype's value within 2 units (CIELAB or degrees) with 95% confidence.

Runner plants from strawberry (Fragaria ×ananassa Duch.) genotypes previously identified as relatively susceptible, intermediate, and resistant to wilt caused by Verticillium dahliae Kreb. were inoculated with a conidial suspension containing a mixture of five isolates of V. dahliae at 10⁴, 10⁵, and 10⁶ conidia/mL. Disease symptoms were scored as the number of dead or severely stunted plants per plot, or on the basis of a subjective phenotypic resistance score assigned to each plot on eight dates during the spring after planting. Overall disease symptoms increased with inoculum concentration; resistance scores for all genotypes were 4.8, 3.7, and 3.2, and the percentages of plants stunted or dead were 6.8, 32.6, and 44.9 for the three conidial concentrations, respectively. The relative resistance categories were separable at concentrations of 10⁶ and 10⁵, whereas no separation was obtained at 10⁴ conidia/mL. Genotypes originally classified as intermediate in resistance performed more like susceptible types at the highest conidial concentration. Significant resistance category × conidial concentration level interactions were detected for resistance score but not for the number of dead or severely stunted plants per plot. Regression coefficients for resistance score and percentage of stunting on conidial concentrations were statistically significant only for susceptible and intermediate genotypes. Some stunting was detected within all resistance categories at the highest conidial concentration, and the practical value of the resistance developed to date will depend ultimately on realistic field inoculum levels.