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White mold, caused by Sclerotinia sclerotiorum (Lib.) de Bary, results in severe losses in the production of common bean, Phaseolus vulgaris, especially snap bean varieties. Advanced backcross QTL analysis was used to identify quantitative trait loci for resistance to white mold in an interspecific cross of P. vulgaris and P. coccineus and to select resistant, well-adapted germplasm for development of superior common bean cultivars. A population of 115 BC2F4 lines from a cross of snap bean cultivar OR 91G x runner bean accession PI 255956 was genotyped using amplified fragment length polymorphism (AFLP®) and simple sequence repeat (SSR) markers. Corresponding BC2F5 progeny were evaluated for resistance to white mold in a greenhouse straw test and for tolerance to oxalate in a laboratory test. BC2F6 lines were then tested for resistance under field conditions. Single factor analysis of variance identified 29 marker loci contributing to response in at least one phenotypic test. One QTL conditioning 6% of the variance for field resistance was identified by composite interval mapping on linkage group 09, anchored to the consensus linkage group b09 by SSR loci. Significant deviation from expected segregation ratios was observed at all but 3 SSR loci and at all but 7 AFLP® loci (p < 0.05). Codominant SSR markers revealed excessive heterozygosity, and an underrepresented donor homozygous marker class. Several SSR markers polymorphic between parents failed to segregate in the progeny, particularly those corresponding to bean core map linkage groups b01, b04, and b05. Several lines were identified as possessing resistance superior to the recurrent parent. These will be incorporated into our efforts to produce highly resistant snap bean varieties. |
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