Fine Mapping of Ur-3, a Historically Important Rust Resistance Locus in Common Bean

Bean rust, caused by Uromyces appendiculatus, is a devastating disease of common bean (Phaseolus vulgaris) in the Americas and Africa. The historically important Ur-3 gene confers resistance to many races of the highly variable bean rust pathogen that overcome other rust resistance genes. Existing molecular markers tagging Ur-3 for use in marker-assisted selection produce false results. Here, we describe the fine mapping of the Ur-3 locus for the development of highly accurate markers linked to Ur-3. An F-2 population from the cross Pinto 114 (susceptible) x Aurora (resistant with Ur-3) was evaluated for its reaction to four different races of U. appendiculatus. A bulked segregant analysis using the SNP chip BARCBEAN6K_3 placed the approximate location of Ur-3 in the lower arm of chromosome Pv11. Specific SSR and SNP markers and haplotype analysis of 18 sequenced bean varieties positioned Ur-3 in a 46.5 kb genomic region from 46.96 to 47.01 Mb on Pv11. We discovered in this region the SS68 KASP marker that was tightly linked to Ur-3. Validation of SS68 on a panel of 130 diverse common bean cultivars containing all known rust resistance genes revealed that SS68 was highly accurate and produced no false results. The SS68 marker will be of great value in pyramiding Ur-3 with other rust resistance genes. It will also significantly reduce time and labor associated with the current phenotypic detection of Ur-3. This is the first utilization of fine mapping to discover markers linked to rust resistance in common bean.