Pathotypes, Distribution, and Metalaxyl Sensitivity of Phytophthora sojae from North Dakota

Phytophthora root rot, caused by Phytophthora sojae, is the most important disease of soybean (Glycine max) in North Dakota. Because of the expansion of soybean hectares and appearance of disease on cultivars with resistance genes, we investigated the pathotypes, distribution, and metalaxyl sensitivity of P. sojae in North Dakota. Soil from 347 soybean fields in 20 counties in eastern North Dakota was collected between 2002 and 2004, and P. sojae was baited from the soil with the susceptible cultivar McCall. The virulence phenotype of each isolate was determined on eight differentials, and all isolates were tested for sensitivity to metalaxyl incorporated into V8 agar. The pathogen was recovered from 80 fields located in five counties. Sixteen pathotypes, which included 14 known races and two previously reported pathotypes that had not been assigned a race, were identified out of 157 isolates. A single pathotype was recovered from 61 fields, 2 pathotypes from 14 fields, 3 pathotypes from 4 fields, and 4 pathotypes from 1 field. Pathotypes with virulence phenotypes 1a,1c,7 (race 4; 39%) and 1a,7 (race 3; 28%) were the most common, representing 67% of the total isolates. One or both of these pathotypes was found in 79% of the fields where P. sojae was recovered. Seven of the 157 isolates showed limited growth on metalaxyl after 14 days of incubation. In the past 10 years, the number of pathotypes of P. sojae in North Dakota has increased from 4 to 16, and pathotypes have developed that can attack the three most common resistance genes found in soybean cultivars for the region.