Phytophthora root and stem rot (PRSR) is a major disease of soybean that can cause yield losses of up to 40 million bushels in a single season in the United States and Canada. In North Dakota, farmers use commercial varieties having the resistance genes Rps 1c, 1k, 3a, and 6, however the study by Yan and Nelson (2019) indicate that there are pathotypes of the PRSR organism, P. sojae, that can defeat these genes. Thus, it is important to evaluate the virulence diversity in the P. sojae population and the effectiveness of Rps genes for long-term disease management. Additionally, there is no information in North Dakota on another PRSR pathogen, P. sansomeana, which has been reported across the Midwest. In this research, we propose to characterize the P. sojae pathotypes, identify and determine the prevalence of P. sansomeana (if present in North Dakota), and screen breeding lines for resistance to P. sojae. For the proposed research, surveys will be conducted to collect soil samples from commercial fields. The causal organisms will be baited using a variety susceptible to P. sojae, isolated, identified and pathotyped. After obtaining P. sojae isolates, lines from NDSU’s soybean breeding program will be screened for resistance to the prevalent pathotypes, race 3 and race 4. Overall, this work will allow for the better understanding of P. sojae populations present in North Dakota and more targeted breeding efforts for resistance to this pathogen, as well as help us provide farmers with effective recommendations of cultivars to plant.

1. The diversity of Phytophthora sojae pathotypes present in North Dakota soybean fields will be determined.
2. Phytophthora sansomeana may be identified in North Dakota soybean fields.
3. The current status on the effectiveness of Rps genes against P. sojae will be determined.
4. The soybean breeding efforts will be improved by targeting the most important pathotypes of P. sojae.
5. Soybean farmers will have access to improved recommendations to manage PRSR, which includes cultivar selection.

• Improved grower recommendations for appropriate cultivar selections
• Improved understanding of P. sojae (and possibly P. sansomeana) populations, which will allow for more targeted soybean breeding efforts

While resistance to P. sojae is present in many commercial soybean cultivars, the understanding of which Rps are the most important for growers to plant is currently lacking. Therefore, this research will allow for the more informed decisions across North Dakota for choosing the proper cultivars. Additionally, this research will assist the soybean breeding efforts at NDSU in ensuring the proper resistance genes are being introgressed into advanced yield lines and future public releases for the farmers.