QTL Discovery and Marker Development for Strawberry Angular Leaf Spot Caused By Xanthomonas fragariae

Combining disease resistance with excellent fruit quality in new rosaceous crop cultivars is the primary goal of the USDA-SCRI-funded RosBREED project. Angular leaf spot (ALS) caused by Xanthomonas fragariae is the only major bacterial disease of cultivated strawberry (Fragaria ×ananassa). While the disease may cause up to 8% reductions of marketable yield in Florida’s winter annual production, no resistant cultivars have been commercialized to date. Introgression of the resistance trait into commercial quality perennial-type germplasm was initiated using two accessions, US4808 and US4809, previously identified as resistant to the four genetic clades of X. fragariae. Previous reports indicated high heritability but the mode of inheritance was undetermined. Resistance to ALS was observed in two years of inoculated field trials that assayed four full-sib families descended from US4808 and US4809. ALS resistance segregated in 1:1 ratio in all families thus indicating control by dominant allele(s) at a single locus. Using a selective genotyping approach with the IStraw90 Axiom^® SNP array and pedigree-based QTL detection software FlexQTL™, a single major-effect QTL was identified in progeny descended from both resistant accessions. Based on SNP probe alignment to the F. vesca sp. bracteata reference sequence, the QTL location was localized at the 33.6 to 33.9 Mb region of LG6D in both resistance sources, and designated as FaRXf1. One high-resolution melting SNP-assay and two simple sequence repeat (SSR) markers with approximately 95% efficiency were developed and will be used for marker-assisted selection in 2016 in the UF strawberry breeding program.