Sources of soybean rust resistance challenged with single-spored isolates of phakopsora pachyrhizi

Soybean rust, caused by the fungus Phakopsora pachyrhizi Syd., is a potentially devastating disease that can cause significant yield losses. Resistance in soybean [Glycine max (L.) Merr.] germplasm, both qualitative and quantitative, may be effective in providing at least partial control of soybean rust. A number of soybean genotypes have resistance to soybean rust, but few of these have been challenged with the recently recovered U.S. isolates. The objective of this study was to evaluate known sources of soybean rust resistance against U.S. isolates of P. pachyrhizi. Twenty-eight soybean genotypes that either contained known major-genes for resistance or had been reported as new sources of soybean rust resistance, along with two susceptible checks, were challenged with six P. pachyrhizi isolates collected in the U.S. All six isolates produced similar phenotypic reactions within each of the genotypes. Five genotypes, including the Rpp1 source and the isoline of 'Williams 82' with Rpp1, had no visible lesions. Eleven genotypes produced red-brown lesions with few uredinia, including the sources of Rpp2 and Rpp3, and the remainder had susceptible tan lesions, including the source of resistance for Rpp4. Uredinial counts from genotypes producing red-brown lesions on live and fixed leaflets showed significant variation in the number of uredinia with a genotype × isolate interaction. Uredinial counts from genotypes producing tan lesions on live and fixed leaflets showed significant variation in the number of uredinia among genotypes, but there was no genotype × isolate interaction. There were significant correlations (r = 0.8, P < 0.0001; and r = 0.4, P < 0.0001) between uredinial counts based on live and fixed leaflets within genotypes producing red-brown lesions and those producing tan lesions, respectively.