RAPD and SCAR Markers Linked to the Ur-6 Andean Gene Controlling Specific Rust Resistance in Common Bean

Bean rust, caused by Uromyces appendiculatus (Pers.: Pers.) Unger, is a major disease of common bean (Phaseolus vulgaris L.). A recommended strategy to obtain durable rust resistance is to use molecular markers linked to rust resistance genes for pyramiding monogenic resistance genes into a single bean cultivar. However, markers tightly linked to Ur-6 of Andean origin conferring specific resistance (SR) to rust present in Olathe have not been reported. Our objectives were to identify random amplified polymorphic DNA (RAPD) markers linked to Ur-6 controlling SR to race 51 using bulked segregant analysis in an F₂ population from the Middle American (MA) common bean cross Olathe (resistant) × Nebr.#1 sel.27 (susceptible), and determine the presence or absence of these identified markers in 70 MA and Andean bean cultivars and breeding lines. A single dominant gene controlling SR to race 51 was found in the F₂ and confirmed in the F₃ Six RAPD markers were detected in a coupling phase linkage with Ur-6 The gene was flanked by two coupling-phase markers OBC06.300 and OAG15.300 at 1.3 and 2.0 cM. Sequence characterized amplified region (SCAR) marker SOBC06.308 was developed based on the specific primer pair designed from the sequence of the RAPD marker OBC06.300. The RAPD and SCAR markers were also present in MA pinto bean germplasm having Ur-6 as well as Golden Gate Wax, another source of Ur-6 However, these markers were also present in six Andean cultivars without Ur-6, suggesting gene pool specificity. Among five repulsion-phase markers identified, OAY15.200 was the most closely linked to Ur-6 at 7.7 cM. We also confirm the Ur-6 location on linkage group 11 of the P. vulgaris map. These RAPD and SCAR markers linked to Ur-6 identified here, along with markers for other independent rust resistance genes, could be utilized to pyramid multiple genes into a bean cultivar for more durable rust resistance.