Common Phytophthora sojae Pathotypes Occurring in South Dakota

Phytophthora root and stem rot, caused by Phytophthora sojae, is an important disease of soybean (Glycine max L.) in South Dakota. Because P. sojae populations are highly diverse and resistance genes deployed in commercial soybean varieties often fail to protect against this pathogen, this study was initiated to determine P. sojae pathotypes occurring in South Dakota. A total of 216 P. sojae isolates were baited from soil collected from 422 soybean fields in 2013 to 2015 and 2017. The pathotype of each isolate was determined by inoculating 10 seedlings of 13 standard soybean P. sojae differential lines using the hypocotyl inoculation technique. Of the 171 pathotyped isolates, 47 unique pathotypes were identified. The virulence complexity of isolates ranged from virulence on one Rps gene (Rps7) to virulence on 13 Rps genes, and mean complexity was 5.2. Harosoy (Rps7), Harlon (Rps1a), Williams 79 (Rps1c), Williams 82 (Rps1k), and Harosoy 13XX (Rps1b) were susceptible to 98, 80, 78, 73, and 72% of the isolates, respectively. These results highlight the highly diverse P. sojae pathotypes in South Dakota and the likely Rps genes to be ineffective in commercial soybean varieties.