New races of Phytophthora infestans rapidly defeat potato late blight (LB) resistance based on Solanum demissum germplasm, and breeders search for new sources of durable LB resistance. We developed and verified six sequence characterized amplified region markers recognizing the race-specific genes R1 and R3 of S. demissum and the broad-spectrum resistance gene RB of S. bulbocastanum and the germplasms of these species and used them to screen 209 accessions of 21 wild Solanum species. In addition to S. demissum, homologues of R1 and R3 were found in several species of series Demissa,Longipedicellata and diploid Tuberosa; R3 homologues were also detected in S. bulbocastanum,S. cardiophyllum and S. ehrenbergii. The RB homologues were found in a wider range of Solanum species. The markers of R1 and R3 genes reliably discerned between germplasms of S. tuberosum ssp. tuberosum and wild sources of LB resistance. Following introgression, the species-specific markers of demissum and bulbocastanum germplasm were rapidly lost, whereas the markers of R1 and R3 genes lasted through several meiotic generations and were maintained at high frequencies in modern potato cultivars. The presence of these markers in demissoid potato cultivars was significantly associated with LB resistance, presuming that both genes contribute to overall defence response.

Stacking (pyramiding) several resistance genes of diverse race specificity in one and the same plant by hybridization provides for high and durable resistance to major diseases, such as potato late blight (LB), especially when breeders combine highly efficient genes for broad-spectrum resistance that are novel to the intruding pathogens. Our collection of potato hybrids manifesting long-lasting LB resistance comprises, as a whole, the germplasm of 26 or 22 Solanum species (as treated by Bukasov and Hawkes, respectively), with up to 8–9 species listed in the pedigree of an individual hybrid. This collection was screened with the markers of ten genes for race-specific resistance to Phytophthora infestans (Rpi genes) initially identified in S. demissum (R1, R2, R3a, R3b, and R8), S. bulbocastanum/S. stoloniferum (Rpi-blb1/ Rpi-sto1, Rpi-blb2, Rpi-blb3) and S. venturii (Rpi-vnt1). The hybrids comprised the markers for up to four-six Rpi genes per plant, and the number of markers was significantly related to LB resistance. Nevertheless, a considerable portion of resistance apparently depended on presently insufficiently characterized resistance genes. Bred from these multiparental hybrids, the advanced lines with the stacks of broad-specificity Rpi genes will help anticipate LB outbreaks caused by rapid pathogen evolution and the arrival of new pathogen strains.

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