2018 ASHS Annual Conference

Gummy stem blight, caused by Didymella bryoniae, is a destructive foliar disease of watermelon in areas with hot and humid climates. Wild watermelon germplasm, PI 189225 is a known source of resistance to gummy stem blight. The identification and the use of the molecular markers linked to resistance genes in the wild-type germplasm will speed up the introgression of the gummy stem blight resistance into new watermelon varieties. An F2 segregating population was obtained from a cross between resistant wild watermelon genotype PI 189225 and susceptible genotype K3, after which genetic analysis of F2- derived F3 families was performed by inoculating plants with a single isolate of D. bryoniae (isolate 002, Jiangsu Academy of Agricultural Sciences). The results of genetic analysis demonstrated that gummy stem blight resistance in PI189225 was controlled by a major QTL, temporarily designated gsb-8.1. Based on the results of BSAseq, two associated regions spanning 0.27 Mb (Chr8: 6,938,347–9,639,798) and 0.57Mb (Chr8:10,358,659–16,101,517) on the short arm of chromosome 8 were identified responsible for the resistance to gummy stem blight using the Δ(SNP-index) method. The result of QTL linkage analysis with KASP SNP markers further mapped the GSB locus between the SNP marker A009383 and SNP marker A009168 at a genetic distance of 0.4 cM and 0.9 cM, respectively. According to the watermelon gene annotation database (http://cucurbitgenomics.org/organism/1), the region contains about 19 annotated genes and out of the 19 genes, two genes showed disease resistance gene analogs, Cla001017 (Cc-nbs-lrr resistance protein) and Cla001019 (pathogenesis-related). This result will facilitate fine mapping and cloning the gsb-8.1 locus. The tightly linked markers for the gsb-8.1 locus will further provide a useful tool for marker-assisted selection of this QTL in watermelon breeding programs.