Phytophthora stem and root rot (PSRR), caused by Phytophthora sojae, is a soilborne oomycete pathogen that ranks among the top five pathogens causing economic losses of soybean annually in the United States (Allen et al. 2017). In Iowa, it is estimated 7.1 million bu were lost to PSRR from 2010 to 2019 (Crop Production Network, 2020). Reducing losses to PSRR relies on single resistance (Rps) genes or partial resistance (governed by many genes) that are deployed in commercial soybean varieties.
Recently Matthiesen et al. (2021) reported the population of P. sojae in Iowa continues to gain virulence on varieties with Rps genes. Furthermore, pathotype complexity (the number of Rps genes an isolate was virulent on) of the isolates had increased since previous surveys. Traditionally it has been postulated that deploying Rps genes exerts selective pressure on the pathogen population so the number of pathotypes within a population increases and/or becomes more complex. In the past 10 years, however, there has been little change in the Rps genes that are deployed in commercial soybean (Matthiesen et al. 2021) and yet the population continues to evolve. Why?
Slusher and St Claire (1973) reported the roots of a cultivars with partial resistance to P. sojae had greater dry weight but as many oospores as the roots of a susceptible cultivar, while the roots of a cultivar with a Rps gene was considerably less oospores. Oospores are sexual thick-walled spores that overwinter in the soil. Thus, planting varieties with partial resistance may be contributing to a buildup of inoculum in the field.
We hypothesize that partial resistance exerts as much or more selection pressure on the population than Rps resistance and is responsible for the increased pathotype complexity in P. sojae reported across the Midwest. Our proposed research will test this hypothesis under controlled conditions. An improved understanding of how resistance in soybean affects P. sojae is crucial to breeders and pathologists to enable improved management of PSRR.

Objective 1: To compare the number of oospores produced on soybean varieties with different types of resistance to P. sojae.
Objective 2: To pathotype oospores of P. sojae recovered from soybean varieties with different types of resistance to P. sojae.

• Enumeration oospores in the roots of PSRR susceptible, resistant and partially resistant varieties
• Improved understanding of contribution of resistance to inoculum levels in the soil.
• Pathotype diversity of oospores recovered from the roots of PSRR susceptible, resistant and partially resistant varieties.
• Validation of molecular method for pathotyping pathotypes of P. sojae.
• Knowledge of the effect of resistance in soybean on pathotype diversity in P. sojae.
• Guidance for soybean breeders developing PSRR resistant cultivars.
• Presentations at various ISUEO extension events (Integrated Crop Management Conference, Crop Advantage Series, Research Farm Field Days) and Crops Team Publications (ICM Newsletter, ICM Blog), and the annual Soybean Breeders workshop.
• Research update reports posted to the National Soybean Checkoff Research Database.

Improved resistant varieties for soybean farmers.
Reduced losses to PSRR and improved profitability for farmers. From 2010-2020, PSRR losses for Iowa were estimated at $0.73 per acre (Crop Protection Network).