QTL for field resistance to late blight in potato are strongly correlated with maturity and vigour

Field resistance to Phytophthora infestans, the causal agent of foliage and tuber blight in cultivated potatoes, earliness (maturity) and vigour, were examined in a diploid segregating potato population grown in replicated tr ials over three consecutive growing seasons. A genetic linkage map of this population was constructed in parallel using PCR-based SSR, AFLP and CAPS m arkers. Analysis of the trait scores alongside the marker segregation data allowed the identification of regions of the genome which were significantl y correlated with components of the respective characters. The most signifi cant associations for all four traits were with marker alleles on potato li nkage group V originating from the male (susceptible) parent. In the case o f foliage resistance to late blight, the positions of the majority of the e ffects, which were located on eleven of the twelve potato linkage groups, h ave been detected in previous [16] and parallel studies [21]. The absence o f Solanum demissum-derived R genes for hypersensitive response to late blig ht and the co-localisation of QTL for resistance, vigour and earliness sugg est that developmental and/or physiological factors play a major role in de termining the level of foliage resistance in this population. In contrast w ith previous findings, a negative correlation was found between foliage and tuber blight resistance.