Three Cycles of Recurrent Maternal Half-Sib Selection Continue to Reduce Foliar Late Blight in a Diploid Hybrid Solanum phureja-S. stenotomum Population

Increasing levels of foliar resistance to late blight, caused by Phytophthora infestans, have previously been reported through two cycles of recurrent maternal half-sib selection in a diploid hybrid population of Solanum phureja-S. stenotomum. The objective of this study was to determine if continued improvements for resistance to late blight could be realized by another cycle of selection. Four clones from each of 72 maternal half-sib families constituting the cycle four population were evaluated for late blight resistance in replicated field trials in Pennsylvania in 2015 and 2016. The experimental design was a randomized complete block design with two replications each year. P. infestans US23 genotype was used in inoculations each year. Area under the disease progress curve (AUDPC) was calculated based on visual assessment of foliar disease five times late in the season each year and subjected to statistical and genetic analyses. Mean AUDPC in the cycle one, two, three, and four populations was 652, 556, 276, and 173, respectively. Narrow-sense heritability for resistance decreased in the cycle four population (h² = 0.54) as compared to prior populations (0.77 ≤ h² ≤ 0.80), however, broad-sense heritability remained high (H = 0.83). Additional improvements for late blight resistance are likely to occur if this approach is continued.