The effect of pyramiding Phytophthora infestans resistance genes R(Pi-mcd1) and R(Pi-ber) in potato

Despite efforts to control late blight in potatoes by introducing R (pi)-genes from wild species into cultivated potato, there are still concerns regarding the durability and level of resistance. Pyramiding R(pi)-genes can be a solution to increase both durability and level of resistance. In this study, two resistance genes, R (Pi-mcd1) and R (Pi-ber), introgressed from the wild tuber-bearing potato species Solanum microdontum and S. berthaultii were combined in a diploid S. tuberosum population. Individual genotypes from this population were classified after four groups, carrying no R (pi)-gene, with only R (Pi-mcd1), with only R (Pi-ber), and a group with the pyramided R (Pi-mcd1) and R (Pi-ber) by means of tightly linked molecular markers. The levels of resistance between the groups were compared in a field experiment in 2007. The group with R (Pi-mcd1) showed a significant delay to reach 50% infection of the leaf area of 3 days. The group with R (Pi-ber) showed a delay of 3 weeks. The resistance level in the pyramid group suggested an additive effect of R (Pi-mcd1) with R (Pi-ber). This suggests that potato breeding can benefit from combining individual R (pi)-genes, irrespective of the weak effect of R (Pi-mcd1) or the strong effect of R (Pi-ber).