FaRXf1: A Locus Conferring Resistance to Xanthomonas fragariae in Octoploid Strawberry

Xanthomonas fragariae, causal agent of bacterial angular leaf spot, is the primary bacterial pathogen of cultivated strawberry (Fragaria ×ananassa Duch.) and is responsible for marketable yield losses of up to 8% in Florida. As there are no publicly available resistant cultivars, and chemical control methods have unwanted side effects, the development of resistance in elite germplasm could be the most effective means to prevent outbreaks. While high levels of resistance have not been found in cultivated strawberry, the octoploid accessions US4808 (F. virginiana) and US4809 [F. virginiana × (F. ×ananassa ‘Earliglow’)] are known to be resistant. The objectives of this work were to introgress resistance from these wild sources into Florida-adapted germplasm, determine the genetic architecture of resistance, and develop a protocol for marker-assisted selection, as outlined by the RosBREED consortium of Rosaceous crop researchers. In previous efforts, the two resistant accessions were crossed to germplasm adapted to high latitudes in a series of modified backcrosses. Two individuals derived from this effort, resistant parents K12-10 and K08-17, were crossed to ‘Strawberry Festival’ to generate two full-sib families named 13.77 (n=137) and 13.78 (n=134), respectively. Progeny were inoculated and phenotyped in the field for resistance to X. fragariae with scores determined as percentage leaf area covered by lesions. Phenotypes segregated in a strongly bimodal fashion, suggesting control of resistance by a dominant allele at a single locus. From the tails of the phenotypic distributions of families 13.77 and 13.78, 36, and 38 individuals, respectively, were genotyped with the Affymetrix IStraw90^® Axiom^® SNP array. Quantitative trait locus analysis was performed with FlexQTL™ software using a novel map of 3,624 SNP markers covering 28 linkage groups. A major locus was detected between 110cM and 118cM spanning an area of 400kb on chromosome 6D in family13.77 and a larger but wholly overlapping region in family 13.78. This novel locus was named FaRXf1, for “Fragaria ×ananassa Resistant to Xanthomonas fragariae 1.” Closely linked molecular markers were developed for use in marker-assisted selection and will accelerate resistance breeding efforts. RosBREED is funded by the USDA-NIFA-SCRI Award number 2014-51181-22378.