White Mold Resistance Derived From An Interspecific Backcross Dry Bean Population.

White mold (WM) disease, caused by Sclerotinia sclerotiorum Lib de Bary, can reduce common bean (Phaseolus vulgaris L.) yield as much as 80% and annual losses in the US are estimated to be as high as 30%. Genetic resistance to WM mold has been reported in both common and scarlet runner (P. coccineus L.) beans. An interspecific inbred backcross line (IBL) population was developed between partially resistant common and scarlet runner bean parents to pyramid resistance genes from each species. Three IBL lines had WM resistance ratings based on the straw test significantly (P<0.05) lower than a well known common bean parent G122 (3.14, 3.29, and 3.32 vs 5.24). Markers linked to QTL on B2b, B7, and B8 linkage groups in common bean and one QTL from P. coccineus accounted for 9.7 (P<0.05), 12.8 (P<0.01), 10.8 (P<0.01), and 7.0% (P<0.05) of the phenotypic variation in resistance, respectively. A total of eleven  molecular markers were found that were linked to resistance in scarlet runner bean parent PI 255956 that accounted for a significant proportion (P<0.05) of the phenotypic variation in resistance. Mean resistance ratings were higher for lines that had >25% scarlet runner alleles compared with lines that had <25% scarlet runner alleles (4.8 vs 3.8, respectively), indicating that lines with higher proportion of scarlet runner alleles also had higher levels of resistance to WM. Our results suggest that MAS for resistance QTL from scarlet runner bean into common bean may be a viable method to pyramid QTL from scarlet runner bean and common bean to improve white mold resistance in common bean.