Epidemic of Phytophthora Root Rot of Soybeans in South Dakota. M. W. Ferguson, Department of Plant Science, South Dakota State University, Brookings 57007. Plant Dis. 71:850. Accepted for publication 14 May 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0850D.
Phytophthora root rot (PRR) of soybeans was first observed in South Dakota in 1981; Phytophthora megasperma Drechs. f. sp. glycinea Kuan & Erwin (Pmg) was isolated (G. Buchenau, unpublished). PRR was not observed again until 1984. Additional isolations of Pmg were made in 1985, but disease incidence did not appear to be increasing. Of the six isolates obtained during 1981-1985, one was race 1, two were race 2, and three were new races (unpublished). In spring 1986, however, rainfall was abnormally high and the incidence of PRR was epidemic. In a 1986 survey of 114 fields in 14 counties of South Dakota, 388 of 433 plant samples (88% of the fields) were positive for Pmg. Preliminary tests indicated that 6, 63, and 31 % of the isolates were race 1,3, and 4, respectively. Plant Disease Clinic (PDC) samples from three additional counties were positive for Pmg. When all 1986 PDC isolates were tested, 33, 39, and 28% were race I, 3, and 4, respectively. Results from both sets of isolates indicate a shift from race 3 to race 4, probably because ofthe amount of material resistant to race 3 that is grown in this region. This suggests that selection pressure for race changes exists, even when symptoms of disease are not observed.