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

Strawberry ( Fragaria × 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     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,     and 0·30, respectively. Most of the selection response obtained through genotypic selection would thus be transferred to segregating offspring.     

Correlation between field disease and latent infection by Glomerella cingulata of strawberry plants as revealed by a simple diagnostic method using ethanol immersion

Leaves of strawberry plants growing in fields were collected and assayed by ethanol immersion treatment (SDEI) to detect latent infection by Glomerella cingulata and Dendrophoma obscurans. SDEI revealed that 83% of the plants in growers fields were latently infected by G. cingulata and 58% by D. obscurans. In such fields, only 0.8%–2.5% of the plants later became wilted or died because of G. cingulata. When the latently infected plants from naturally infested fields were kept under optimal conditions for disease development for 5 weeks, 70.0%–83.3% of them wilted or died from G. cingulata. However, only 5.6%–6.7% of the plants diagnosed by SDEI as being without latent infection wilted or died. The results indicate the importance of selecting healthy plants, suggesting that SDEI is an effective method for detecting latent infection and reducing losses from G. cingulata.