Mapping of the Ur-7 gene for specific resistance to rust 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 linked to the Ur-7 gene for specific resistance (SR) to rust present in the cultivar Great Northern (GN) 1140 have not been reported. Our objectives were to identify random amplified polymorphic DNA (RAPD) markers linked to the Ur-7 gene for SR to rust race 59 using bulked segregant analysis in an F-2 population from the Middle American (MA) common bean cross 'GN1140' (resistant) X GN Nebr. #1 (susceptible) and to map the Ur-7 gene on an existing RAPD marker-based linkage map constructed by means of recombinant inbred lines (RILs) from the MA cross GN BelNeb-RR-1 X A 55. A single dominant gene controlling SR to race 59 was found in the F-2 and confirmed in the F-3. Six RAPD markers were detected in a coupling phase linkage with the Ur-7 gene. Cosegregating coupling-phase markers OAD12.550 and OAF17.900 were found. These were also present in pinto US-5 from which the rust resistance of GN1140 was derived. Among the three repulsion-phase markers, marker OAB18.650 was the most closely linked to the Ur-7 gene at a distance of 7.6 centimorgans (cM). All linked markers detected in the F2 population also segregated in the RILs and were located on linkage group 11 of the existing linkage map. These markers linked to the Ur-7 gene of MA origin identified here, along with other independent rust resistance genes from other germplasm, could be utilized to pyramid multiple genes into a bean cultivar for more durable rust resistance.