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Comparison of Pathogenic Variation among Phakopsora pachyrhizi isolates collected from the United States and International Locations, and Identification of Soybean Genotypes Resistant to the U.S. Isolates

Paul, C., Frederick, R. D., Hill, C. B., Hartman, G. L., Walker, D. R.
Plant disease 2015 v.99 no.8 pp. 1059-1069
Glycine max, Phakopsora pachyrhizi, disease resistance, genotype, germplasm, greenhouse experimentation, plant breeding, principal component analysis, signs and symptoms (plants), soybean rust, soybeans, virulence, United States
A major constraint in breeding for resistance to soybean rust has been the virulence diversity in Phakopsora pachyrhizi populations. In a greenhouse experiment, reactions of 18 soybean genotypes to 24 U.S. isolates collected 2007-2008 and four foreign isolates were compared. Reactions of four differentials to these U.S. isolates were also compared with reactions to nine foreign isolates and three U.S. isolates from 2004. Principal component analysis (PCA) of the infection types grouped the U.S. isolates into a single virulence group, whereas each of the foreign isolates had a unique virulence pattern. In another experiment, reactions to the 24 U.S. isolates were compared using a set of 11 soybean differentials. There were significant interactions (P < 0.001) between the isolates and host genotypes for rust severity and uredinia densities, and the 24 isolates were assigned to either seven or six aggressiveness groups from PCA of the two respective measures of disease. Evaluation of 20 soybean genotypes for resistance to the previously established aggressive groups identified 10 genotypes resistant to isolates representing most of the groups. This study confirmed the pathogenic diversity in P. pachyrhizi populations and identified soybean germplasm with resistance to representative U.S. isolates that can be used in breeding.