Evaluation of Soybean Germplasm for Resistance to Phakopsora pachyrhizi

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Abstract
Phakopsora pachyrhizi, the causal fungus of soybean rust, was discovered in the continental U.S. in November 2004. The presence of this disease in the U.S. may have an impact on soybean (Glycine max) production, as the current commercial varieties are considered to be susceptible, and the use of one or more applications of fungicides will add additional costs to production. One objective of the USDA-ARS research on soybean rust is to identify soybean germplasm with resistance to the disease. There are over 16,000 soybean accessions in the USDA Germplasm Collection located at the University of Illinois. These accessions were evaluated in a two-tiered inoculation program using a mixture of four P. pachyrhizi isolates in Biosafety Level 3 containment greenhouses the FDWSRU. In the first round of evaluations, 16,595 accessions were rated for rust severity. Of these, 3,215 accessions, based on low visual rust severity or the presence of a red-brown reaction, were selected for a second round of evaluation. After the second round of replicated evaluations of the 3,215 accessions, 805 were selected for further evaluation, again based on low mean visual severity or the presence of a red-brown reaction. Some of these selected accessions have the potential to provide soybean rust resistance genes that may be useful for incorporation into commercial soybean cultivars. Accepted for publication 9 November 2005. Published 4 January 2006.