Soilborne Oomycete Population Structure in Soybean ( Glycine max ) Fields and Historical Deployment of Rps Genes in Commercial Soybean Cultivars in Wisconsin

The United States plays a significant role in fulfilling global soybean demand. Recent weather and changing management practices have increased oomycete-induced diseases in soybean fields in the United States and, more specifically, in Wisconsin. This study aimed to assess the oomycete diversity and abundance in soybean-growing regions in Wisconsin, characterize the pathotypes of Phytophthora sojae, and compare them with the soybean Rps genes historically deployed in Wisconsin’s commercial cultivars. Over 2 years, 274 soil samples from 39 counties were successfully baited for oomycetes using a soybean leaf disc assay. In total, 388 isolates were identified from four oomycete genera. Overall, Globisporangium was the most abundant genus detected. Although a low diversity was observed in the samples, a high diversity was observed across counties, likely influenced by their geographic locations and the years sampled. While P. sojae was isolated at low frequency, the hypocotyl pathotyping assay identified two pathotypes with alleles 1a, 1b, 1c, 1k, and 3b. Analysis of the deployed Rps genes in variety trials in Wisconsin from 2015 to 2024 indicated a high frequency of the Rps1c and Rps1k genes in commercial cultivars. The pathotypes detected in this study suggest that the deployment of these genes is no longer effective, so the use of cultivars with the Rps3a gene is highly recommended. The insights regarding the oomycete population diversity and abundance in the state will enable the deployment of targeted disease management, addressing yield reductions caused by pathogenic oomycete species.