Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2009
Publication Date: 5/27/2009
Citation: Paul, C., Hartman, G.L. 2009. Sources of Soybean Rust Resistance Challenged with Six Purified Phakopsora Pachyrhizi Isolates Collected from the USA. Crop Science. 49:1781-1785.
Interpretive Summary: Soybean rust is caused by a fungus that is widely distributed in soybeans grown in sub-tropicical areas. Over the last decade, there has been a concerted effort to find additional sources of resistance to soybean rust. In this study, six isolates of the fungus were used to challenge 28 soybean genotypes that had been previously reported as being resistant to the fungus. Soybean genotypes differed in their responses with some being highly resistant to all six isolates, while others were highly susceptible. Only 11 of the 28 genotypes tested produced a resistant reaction with little or no fungal reproduction. This is the first report to challenge many of the known and new sources of soybean rust resistance to purified domestic USA-collected isolates of the fungus. This information is important to soybean pathologists and breeders that are working on research to improve resistance to soybean rust.
Technical Abstract: Soybean rust, caused by the fungus Phakopsora pachyrhizi, is a potentially devastating disease that can cause significant yield losses. Resistance, both qualitative and quantitative, in soybean germplasm may be effective in at least providing partial control of soybean rust. There are a number of soybean genotypes with resistance to soybean rust, but few of these have been challenged to purified domestic isolates in the USA. The objective was to determine if these reported resistance sources would be resistant to USA collected isolates. Twenty eight soybean genotypes, that either contained known major genes for resistance or were reported as new sources of soybean rust resistance, were challenged to six P. pachyrhizi isolates collected in the USA. 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 had reddish 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. Uredinia counts on live and fixed leaflets showed significant variation in the number of uredinia by genotype, but there was no genotype by isolate interaction. There was a significant correlation (r = 0.78, P < 0.0001) between uredinia counts based on live and fixed leaflets. This is the first report to challenge many of the known and new sources of soybean rust resistance to purified domestic isolates of P. pachyrhizi.