Disease Note.

Races of Phytophthora sojae on Soybean in the Red River Valley of Minnesota and North Dakota. B. D. Nelson, Department of Plant Pathology, North Dakota State University, Fargo 58105 . J. M. Hansen, Department of Plant Pathology, North Dakota State University, Fargo 58105 and C. E. Windels, Northwest Experiment Station, University of Minnesota. Crookston 56716. Plant Dis. 80:104. Accepted for publication 6 November 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0104A.

Phytophthora root and stem rot of soybean (Glycine max (L.) Merr.) caused by Phytophlhora sojae M. J. Kaufmann & J. W. Gerdemann is becoming important in the Red River Valley (RRV) of Minnesota and North Dakota. High rainfall and flooding during 1993 and 1994 increased severity and incidence of disease producers reported extensive damage from P. sojae in the southern RRV in 1994. Races of P. sojae in the RRV have not been characterized previously. Soybeans with symptoms of Phytophthora root and stem rot were collected at random from the RRV between 1991 and 1994. Phytophlhora sojae was isolated from stems placed on a selective medium (1) cultures were stored on V-8 agar in sterile distilled water. Eighty-one isolates from 51 fields were tested for virulence using the hypocotyl inoculation method (2) on eight isolines of Harosoy containing genes (rps, Rps1-a + Rps 7, Rps1-b, Rps1-c, Rpsl-d, Rps1-k + Rps 7. Rps3-a + Rps 7, and Rps 6 + Rps 7) for resistance to P. sojae. A known isolate of race 3 was used as a control and all tests were repeated. Virulence patterns for races 1, 3, 4, and 9 were identified for 4, 53, 4, and 3 of the isolates, respectively. Seventeen isolates were not classified because of inconsistent reactions. Only one race was obtained from each field with two exceptions races 3 and 4 and races 1 and 3 occurred together in one and two fields, respectively. Although race 3 predominates in the RRV. most adapted soybean cultivars have no Rps genes for control of this race.