Race Survey of Phytophthora Sojae, Causal Agent of Phytophthora Root Rot of Soybean, in Kentucky

Phytophthora root rot (caused by Phytophthora sojae) can be a destructive disease of soybean in years in which rainfall and wet soils are abundant. Management of this disease is accomplished primarily through the use of resistant varieties that utilize either single resistant genes (known as Rps genes) or partial resistance (sometimes referred to as “field resistance” or “tolerance”). The use of Rps genes over time has allowed for the selection of races of P. sojae that can overcome specific Rps genes, rendering these resistant genes ineffective. Understanding the diversity of P. sojae races in Kentucky can help soybean farmers make better choices when selecting varieties, and can help soybean breeding programs make better choices when determining which Rps genes to utilize when developing new varieties for Kentucky farmers. The last P. sojae race survey conducted in Kentucky was in 1994 (Hershman and Abney, 1997). In that survey, race 1 was the predominant race present in Kentucky soils. Race 1 is able to overcome the Rps7 gene, which is rarely present in modern soybean varieties grown in the state. In over 200 soybean varieties tested by the University of Kentucky Soybean Variety Performance Testing Program in 2016, approximately 38% of those varieties had the Rps1c gene, 9% had the Rps1a gene, and 8% had the Rps1k gene. The use of these genes for several years, likely has led to the selection of new virulent races of P. sojae in Kentucky, but no information is currently available to confirm that. Results from a P. sojae race survey that occurred in several North Central states in 2012 and 2013 (including Kentucky’s three bordering states to the north – Illinois, Indiana, and Ohio), showed that over 40% of the P. sojae isolates recovered were virulent against (could overcome) Rps1a, Rps1c, or Rps1k (Dorrance et al., 2016). Based on these results, it is highly likely that race shifts also have occurred in Kentucky. The objective of this proposed research is