RB and Ph resistance genes in potato and tomato minimize risk for oospore production in the presence of mating pairs of Phytophthora infestans

Late blight caused by the heterothallic oomycete Phytophthora infestans, is a constraint of potato and tomato production worldwide. Currently, pathogen populations within the U.S. are primarily asexual with limited recombination or soil persistent oospores. Both mating types, however, were isolated from Wisconsin during 2009, 2010, and 2013, posing risk for oospores. Late blight resistance genes can be useful in disease reduction against currently predominant U.S. clonal lineages, however, resistance is not complete. To assess disease and oospores with US-22 (A2), US-23 (A1), and US-24 (A1) clonal lineages, fifteen solanaceous hosts with different resistance genetics were investigated. Potato and tomato transformed with the RB gene and tomato ‘Mountain Magic,’ carrying Ph-2 and Ph-3, were the most tolerant to all pathogen lineages with significantly less disease severity (<30%) than the other twelve varieties (40–100% severity). Oospores formed in plant tissues after inoculation with both mating pairs on susceptible and moderately resistant hosts at 12–20 °C, with the greatest number of oospores at 16 °C. The US-24 x US-22 cross produced significantly more oospores than the US-23 x US-22 cross. No oospores were detected in tomato with Ph2 and Ph3, or potato and tomato with RB using whole plant assays. Varieties with these genes could play an important part of an integrated late blight management program by limiting disease and the sexual phase of the pathogen. Deployment of Ph and RB could greatly reduce the risk of oospores and sexual recombination, thereby reducing population variation and development of soilborne P. infestans.