Components of genetic variation for resistance of strawberry to Phytophthora cactorum estimated using segregating seedling populations and their parent genotypes

Strawberry (Fragaria x ananassa) seedlings from 50 bi-parental crosses among 20 elite genotypes were evaluated for resistance to Phytophthora cactorum after artificial inoculation. Plots of seedlings or runner plants were rated using a disease severity score and the percentage of stunted plants per plot. The distribution of cross means for percentages of plants with stunting was highly skewed; 79% of the inoculated seedlings showed some level of stunting compared to non-inoculated control seedlings, and all but one of the crosses had 50% or more stunted plants. Disease severity scores for the bi-parental crosses were normally distributed and expressed a range of variation not reflected by the percentage of visibly stunted plants. Factorial analysis based on seedling plot means demonstrated significant additive genetic variance for the disease severity score, and the additive genetic variance was 1.9 times greater than the estimated dominance variance. The cross-mean heritability was h(c)(2) = 0.63 for the severity score. Estimates of the additive genetic variance component using the covariance of severity scores obtained from the seedling analysis and with severity scores for their parents evaluated in a commercial environment were similar, sigma(2)(a) = 0.39 and 0.30, respectively. Most of the selection response obtained through genotypic selection would thus be transferred to segregating offspring.