Identification and mapping of a powdery mildew resistance gene Vu-Pm1 in the Chinese asparagus bean landrace ZN016

Powdery mildew (PM), caused by the fungus Erysiphe polygoni D.C, is one of the most common and serious diseases of cowpea (Vigna. unguiculata L. Walp.)/asparagus bean (Vigna. unguiculata ssp. sesquipedialis). Deployment of scab-resistant varieties is the major strategy to curb this disease. ZN016, a Chinese asparagus bean landrace, is highly resistant to powdery mildew. To identify the resistance genes in ZN016, an F_7:8 recombinant inbred line (RIL) population derived from the cross between ZN016 and a susceptible cultivar Zhijiang 282 was evaluated for PM resistance in the open field in 2008 and 2009. Phenotypic analysis indicated that ZN016 harbors a single PM resistance gene. Using a SNP and SSR based genetic map constructed with this population, the PM resistance gene was mapped to a locus on LG9 flanked by SSR markers CLM0305 and CLM0260, with a genetic distance of 2.0 cM and 5.0 cM, respectively. To our knowledge, this is the first powdery mildew resistance gene reported in cowpea, which was temporarily designated as Vu-Pm1. The identification of Vu-Pm1 opens a door for dissecting resistance mechanisms and applying marker-assisted selection for the development of cultivars with PM resistance in cowpea.