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Genetic Mapping of Ph-2, a Single Locus Controlling Partial Resistance to Phytophthora infestans in Tomato

Late blight caused by the fungal pathogen Phytophthora infestans is one of the most important diseases of potato (Solanum tuberosum) and tomato (Lycopersicon esculentum). Genetic analysis of the resistance to this pathogen was performed on an F₂ progeny of 322 plants derived from a cross between the tomato line L. esculentum var. Hawaii7996 susceptible to late blight and the resistant wild relative L. pimpinellifolium WVa700. Qualitative and quantitative analyses of the resistance with restriction fragment length polymorphism markers spanning the genome showed that this resistance was controlled by a single, incompletely dominant allele, Ph-2, present on the distal part of the long arm of chromosome 10 in an interval of 8.4 cM flanked by markers CP105 and TG233. Genetic analysis of F₂ progeny from a second cross between an L. esculentum introgression line IL10-3 carrying a homozygous L. pennellii segment spanning the distal part of the long arm of chromosome 10 and WVa700 confirmed the map location, but high suppression of recombination was observed in this cross in the introgressed fragment. A high-resolution genetic linkage map of the chromosomal region surrounding Ph-2 was initiated to permit future map-based cloning of this gene. Amplified fragment length polymorphism markers closely linked to Ph-2 were screened by bulked segregant analysis.