Comparison of Phytophthora sojae populations in Iowa and Nebraska to identify effective Rps genes for Phytophthora stem and root rot management

Abstract

Phytophthora stem and root rot (PSRR) of soybean, caused by the oomycete Phytophthora sojae, is prevalent in Iowa and Nebraska. Reducing losses to PSRR primarily relies on growing cultivars with specific resistance (Rps) genes. Predominant genes used in commercial soybean cultivars include Rps1a, Rps1c, Rps1k, and Rps3a. Knowledge of which Rps gene to deploy depends on understanding which genes remain effective against local pathogen populations. From 2016 to 2018, 326 isolates of P. sojae were recovered from fields in Iowa and Nebraska and classified into pathotypes based on their virulence on 15 soybean genotypes. A total of 15 and 10 pathotypes were identified in Iowa and Nebraska, respectively. Almost all isolates were virulent on Rps1a, and more than 70% of isolates were virulent on Rps1c and Rps1k. Only 2.3% of isolates from Iowa were virulent on Rps3a. Among commercial soybean cultivars evaluated in the Illinois Soybean Variety Trials from 2010 to 2020, Rps1c was the most frequently reported gene, followed by Rps1k. In contrast, Rps1a and Rps3a were present in fewer than 10% and fewer than 5% of cultivars, respectively. Because many P. sojae isolates were virulent on Rps1a, Rps1c, and Rps1k, soybean cultivars carrying these genes are unlikely to provide adequate protection against PSRR unless combined with high levels of partial resistance.