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Strains of Soybean Mosaic Virus: Classification Based on Virulence in Resistant Soybean Cultivars. Eui- Kyoo Cho, Graduate student, Department of Plant Pathology and International Soybean Program (INTSOY), University of Illinois, Urbana, 61801; Robert M. Goodman, associate professor, Department of Plant Pathology and International Soybean Program (INTSOY), University of Illinois, Urbana, 61801. Phytopathology 69:467-470. Accepted for publication 8 November 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-467.

Ninety-eight isolates of soybean mosaic virus (SMV) from seeds in the USDA soybean germplasm collections were classified into seven strains based on disease reactions of inoculated differential soybean cultivars. Two susceptible soybean cultivars (Clark and Rampage) and six putatively resistant cultivars (Buffalo, Davis, Kwanggyo, Marshall, Ogden, and York) were used. The results showed differences in virulence among SMV strains and in the susceptibility and reactions of soybean cultivars to these strains. Cultivars resistant to less virulent strains in most cases exhibited severe necrotic symptoms when inoculated with more virulent strains. All SMV strains tested caused infection and typical mosaic symptoms in cultivars Clark and Rampage. Strain G1 (including ATCC PV-94) did not infect any of the resistant cultivars. Strain G2 caused local and systemic necrosis in Marshall but did not infect other resistant cultivars. Strain G3 and strain G4 caused local and systemic necrosis in both Ogden and Marshall; strain G4 also infected Davis and York, causing either local and systemic necrosis or mosaic symptoms. Strains G5, G6, and G7 all caused mosaic symptoms in Davis and York; strain G5 also caused necrosis in Kwanggyo, strain G6 caused necrosis in both Kwanggyo and Marshall, and strain G7, which infected all cultivars tested, caused necrosis in Marshall, Odgen, Kwanggyo, and Buffalo. The incidence of SMV and the presence of highly virulent strains in germplasm collections call for use of a range of SMV strains in soybean breeding programs in which SMV resistance is an objective.