Identification and mapping of late blight resistance quantitative trait loci in tomato accession PI 163245

Late blight (LB), caused by the oomycete Phytophthora infestans (Mont.) de Bary, is one of the most devastating diseases of tomato (Solanum lycopersicum L.) and potato (S. tuberosum L.) worldwide. The importance of LB on tomato has increased due to the occurrence of aggressive and fungicide-resistant clonal lineages of P. infestans. Consequently, identification and characterization of new sources of genetic resistance to LB has become a priority in tomato breeding. Previously, we reported accession PI 163245 as a promising source of highly heritable LB resistance for tomato breeding. The purpose of this study was to identify and map quantitative trait loci (QTLs) associated with LB resistance in this accession using a trait-based marker analysis (a.k.a. selective genotyping). An F₂ mapping population (n = 560) derived from a cross between a LB-susceptible tomato breeding line (Fla. 8059) and PI 163245 was screened for LB resistance, and the most resistant (n = 39) and susceptible (n = 35) individuals were selected for genotyping. Sequencing and comparison of the reduced representation libraries (RRLs) derived from genomic DNA of the two parents resulted in the identification of 33,541 putative single nucleotide polymorphism (SNP) markers, of which, 233 genome-wide markers were used to genotype the 74 selected F₂ individuals. The marker analysis resulted in the identification of four LB resistance QTLs conferred by PI 163245, located on chromosomes 2, 3, 10, and 11. Research is underway to develop near-isogenic lines (NILs) for fine mapping the QTLs and develop tomato breeding lines with LB resistance introduced from PI 163245.