Disease Note

Root Rot of Soybean in Minnesota: The Changing Race Situation in Phytophthoramegasperma f. sp, glycinea. B. W. Kennedy, Department of Plant Pathology, University of Minnesota, St. Paul 55108. Plant Dis. 68:826. Accepted for publication 19 June 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-826f.

Race I of Phytophihora megasperma Drechs. f. sp. glycinea Kuan & Erwin (Pmg) was first discovered in Minnesota in 1964 on Glycine max L. and remained stable for many years. Race 3 was isolated in 1979 when a commercial field of the cultivar Steele, resistant to race 1, was damaged by root rot. During 1980- 1982, scattered occurrences of race 3 were noted in commercial plantings. In 1983, 36 of 41 samples of diseased plants received from widely scattered commercial fields in southern and south central Minnesota had typical Pmg symptons. Of the 20 cultures obtained, one was race 1, eleven were race 3, seven were race 4, and one was race 6. An estimated 90% of commercial plantings in the state in 1983 were resistant to race 1, and it is apparent that other races to which certain commercial cultivars are susceptible are becoming more prevalent.