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 Rps 1a, Rps 1c, Rps 1k, and Rps 3a. Knowing which Rps gene to deploy depends on knowledge of which genes are effective against the pathogen. From 2016 to 2018, 326 isolates of P. sojae from 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 Rps 1a, and over 70% of isolates were virulent on Rps 1c and Rps 1k. Only 2.3% of isolates from Iowa were virulent on Rps 3a. Among commercial soybean cultivars tested in the Illinois Soybean Variety trials from 2010 to 2020, Rps 1c was always the most frequently reported gene, followed by Rps 1k. In contrast, Rps 1a and Rps 3a were present in less than 10% and less than 5% of the cultivars tested, respectively. Because many of the P. sojae isolates in our study were virulent on Rps 1a, Rps 1c, and Rps 1k, soybean cultivars with these genes are unlikely to provide protection against PSRR unless they have a high level of partial resistance.