Genetic map of the powdery mildew resistance gene in soybean PI 243540.

Powdery mildew (caused by Microsphaera diffusa Cooke & Peck) is a common disease of soybean in many soybean-growing regions of the world and under greenhouse conditions. The previously reported Rmd locus of soybean for resistance to powdery mildew was mapped on soybean molecular linkage group J (chromosome 16). We have discovered a single dominant gene in PI 243540 that provides season-long resistance to powdery mildew. The objective of this study was to map the powdery mildew resistance gene in PI 243540 with PCR-based molecular markers. One hundred eighty-four F2 plants and their F(2:3) families from a cross between the powdery mildew susceptible cultivar 'Wyandot' and PI 243540 were screened with M. diffusa in greenhouses. Bulked segregant analysis (BSA) with SSR markers was used to identify the tentative genomic location of the gene. The BSA localized the gene to a genomic region in soybean chromosome 16. A linkage map with seven SSR and six SNP markers flanking the gene was constructed. We positioned the gene between SSR marker Sat_224 and SNP marker BARC-021875-04228 at distances of 9.6 and 1.3 cM from the markers, respectively. The map position of the gene was slightly different from previously reported map positions of the only known Rmd locus. We have mapped a single dominant gene, tentatively called Rmd_PI243540, near the previously known Rmd locus on chromosome 16. The molecular markers flanking the gene will be useful for marker-assisted selection of this gene.