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.

A coiled coil-nucleotide binding site-leucine-rich repeat gene RB/Rpi-blb1 isolated from Solanum bulbocastanum confers broad-spectrum resistance to Phytophthora infestans and is currently employed in potato breeding for durable late blight (LB) resistance. RB homologues were reported in several Solanum species; some of them retained defence function. Here, we report additional evidence on RB-like sequences in 21 Solanum species of the section Petota. The panel of Solanum species was screened with three RB-related PCR markers. RB-like sequences were found in every tested Solanum accession, suggesting universal distribution of RB structural homologues among Solanum genomes, while locus-specific RB-629 was found only in 15 species. Phylogenetic analysis of RB-629 sequences suggested a highly conservative pattern of polymorphisms that was neither species- nor series-specific. Apparently, duplication and evolution of RB-like loci preceded Solanum speciation. Marker presence and particular haplotypes were not immediately associated with high LB resistance.

(1) Background: Although resistance to pathogens and pests has been researched in many potato cultivars and breeding lines with DNA markers, there is scarce evidence as to the efficiency of the marker-assisted selection (MAS) for these traits when applied at the early stages of breeding. A goal of this study was to estimate the potential of affordable DNA markers to track resistance genes that are effective against the pathogen Phytophthora infestans (Rpi genes), as a practical breeding tool on a progeny of 68 clones derived from a cross between the cultivar Sudarynya and the hybrid 13/11-09. (2) Methods: this population was studied for four years to elucidate the distribution of late blight (LB) resistance and other agronomical desirable or simple to phenotype traits such as tuber and flower pigmentation, yield capacity and structure. LB resistance was phenotypically evaluated following natural and artificial infection and the presence/absence of nine Rpi genes was assessed with 11 sequence-characterized amplified region (SCAR) markers. To validate this analysis, the profile of Rpi genes in the 13/11-09 parent was established using diagnostic resistance gene enrichment sequencing (dRenSeq) as a gold standard. (3) Results: at the early stages of a breeding program, when screening the segregation of F1 offspring, MAS can halve the workload and selected SCAR markers for Rpi genes provide useful tools.

Late blight (LB) caused by the oomycete Phytophthora infestans (Mont.) de Bary is the greatest threat to potato production worldwide. Current potato breeding for LB resistance heavily depends on the introduction of new genes for resistance to P. infestans (Rpi genes). Such genes have been discovered in highly diverse wild, primitive, and cultivated species of tuber-bearing potatoes (Solanum L. section Petota Dumort.) and introgressed into the elite potato cultivars by hybridization and transgenic complementation. Unfortunately, even the most resistant potato varieties have been overcome by LB due to the arrival of new pathogen strains and their rapid evolution. Therefore, novel sources for germplasm enhancement comprising the broad-spectrum Rpi genes are in high demand with breeders who aim to provide durable LB resistance. The Genbank of the N.I. Vavilov Institute of Plant Genetic Resources (VIR) in St. Petersburg harbors one of the world’s largest collections of potato and potato relatives. In this study, LB resistance was evaluated in a core selection representing 20 species of seven Petota series according to the Hawkes (1990) classification: Bulbocastana (Rydb.) Hawkes, Demissa Buk., Longipedicellata Buk., Maglia Bitt., Pinnatisecta (Rydb.) Hawkes, Tuberosa (Rydb.) Hawkes (wild and cultivated species), and Yungasensa Corr. LB resistance was assessed in 96 accessions representing 18 species in the laboratory test with detached leaves using a highly virulent and aggressive isolate of P. infestans. The Petota species notably differed in their LB resistance: S. bulbocastanum Dun., S. demissum Lindl., S. cardiophyllum Lindl., and S. berthaultii Hawkes stood out at a high frequency of resistant accessions (7–9 points on a 9-point scale). Well-established specific SCAR markers of ten Rpi genes—Rpi-R1, Rpi-R2/Rpi-blb3, Rpi-R3a, Rpi-R3b, Rpi-R8, Rpi-blb1/Rpi-sto1, Rpi-blb2, and Rpi-vnt1—were used to mine 117 accessions representing 20 species from seven Petota series. In particular, our evidence confirmed the diverse Rpi gene location in two American continents. The structural homologs of the Rpi-R2, Rpi-R3a, Rpi-R3b, and Rpi-R8 genes were found in the North American species other than S. demissum, the species that was the original source of these genes for early potato breeding, and in some cases, in the South American Tuberosa species. The Rpi-blb1/Rpi-sto1 orthologs from S. bulbocastanum and S. stoloniferum Schlechtd et Bché were restricted to genome B in the Mesoamerican series Bulbocastana, Pinnatisecta, and Longipedicellata. The structural homologs of the Rpi-vnt1 gene that were initially identified in the South American species S. venturii Hawkes and Hjert. were reported, for the first time, in the North American series of Petota species.