Association Mapping for Fusarium Wilt Resistance in Chinese Asparagus Bean Germplasm

Fusarium wilt (FW) is an important vascular disease attacking asparagus bean [Vigna unguiculata (L.) Walp. subsp. unguiculata Sesquipedalis Group] in China. The level and genetic variability of FW resistance in the Chinese asparagus bean germplasm remains elusive. In the current study, FW resistance was assessed across a natural population consisting of 95 asparagus bean and four African cowpea [Vigna unguiculata (L.) Walp. subsp. unguiculata Unguiculata Group] accessions. The disease index (DI) based on the severity of leaf damage (LFD) and vascular discoloration (VD) varied highly across the population and the highly resistant varieties used for vegetable are very limited. Genome-wide association study identified 11 and seven single nucleotide polymorphisms (SNPs) that are associated with LFD and VD traits, respectively. These SNPs were distributed on nine linkage groups of the asparagus bean genome and each accounted for less than 5% of the phenotypic variation. Overall, the nonstandard vegetable (NSV) subgene pool harbors favorable alleles in a higher frequency than the standard vegetable (SV) subgene pool. Individual NSV-type accessions tend to possess a greater number of favorable alleles than the SV-type ones. A SNP marker 1_0981 was converted to a cleaved amplified polymorphic sequences (CAPS) marker to facilitate future breeding. To our knowledge, this is the first report of an association mapping (AM) study in asparagus bean. The results obtained suggests that resources for FW resistance is relatively limited in the SV subgene pool; hence, introducing resistant alleles from the NSV accessions into currently leading SV cultivars will be important to improve FW resistance of the latter.