Identifying Sources of Partial Resistance and Tolerance to Phytophthora and Pythium Root Rots

Phytophthora and Pythium root rots are destructive diseases of soybean in short-season regions. Breeding for resistance is the most practical, economical, and environmentally safe measure against these diseases. We screened 87 new and old short-season soybean varieties for resistance to three common races of Phytophthora sojae (Races 1, 3, and 5) and for partial resistance (multi-gene resistance) and tolerance to Phytophthora and Pythium root rots in Ontario. Of the 87 varieties evaluated to P. sojae, 29 were resistant to at least one of the three races.  Cultivars AC Brant, AC Orford, Beechwood, Maple Arrow, Maple Donovan, and OAC Gretna were resistant to all three races.Twelve of the 27 partially resistant cultivars were further selected for field trials at Harrow and Ottawa during 2009, 2010, and 2011.  All of the 12 cultivars showed consistent high levels of partial resistance with plant mortality ranging from 9.5 to 16.7%, which were significantly lower than the susceptible control cultivar (OX20-8, 56.1% plant mortality).   Yield reductions of the 12 cultivars ranged from 29.4 to 58.4% on average of three years. AC Bravor, Maple Donovan, Renfrew and AC Hercule had yield reduction of 29.4, 32.1, 37.0, and 37.4%, respectively, and were more tolerant than the remaining cultivars (39.2-58.4%). These cultivars can be used as sources of partial resistance and tolerance to Phytophthora root rot for future cultivar development for Canada.  A total of 94 soybean cultivars were screened in both laboratory and greenhouse trials under artificial inoculations with a highly pathogenic strain of Pythium ultimum var. ultimum isolated from a diseased soybean plant in Ontario in 2010.  None of the 94 cultivars were resistant to either Pythium seed rot or Pythium seedling blight.  Cultivars Maple Arrow Brown and Maple Ridge Brown showed moderate levels of resistance to these diseases.