Mapping QTLs Associated with Resistance to Bacterial Spot (Xanthomonas arboricola pv. pruni) in Peach

Bacterial spot, caused by Xanthomonas arboricola pv. pruni (Xap), is a serious disease that can affect peach fruit quality and production worldwide. This disease causes severe defoliation and blemishing of fruit, particularly in areas with high rainfall, strong winds, high humidity and sandy soil. The molecular basis of its tolerance and susceptibility in peach is yet to be understood. To study the genetics of the peach response to Xap, an F₂ segregating population between two peach cultivars, 'Clayton', a resistant phenotype, and 'O’Henry', which is very susceptible to Xap, was created. Phenotypic data for leaf and fruit response to Xap infection were collected over three years at two locations: the Sandhills Research Station, Jackson Springs, North Carolina (NC); the Sandhill Research and Education Center, Pontiac, South Carolina (SC). Sixty-three individuals exhibiting high tolerance/resistance to Xap were genotyped with an IPSC 9K peach SNP array v1. Out of 8,144 SNPs 1,341 were used to construct a genetic linkage map. This map covers a genetic distance of 421.4 cM with an average spacing of 1.6 cM and is used for mapping QTLs responsible for Xap in peach. The order of 95% of the mapped SNP markers was consistent with the marker order on the peach genome v1.0 assembly. A QTL analysis revealed 14 QTLs involved in Xap resistance: 3 on linkage group (LG) 1; two each on LG2, 3, 4 and 8; and one each on LG5, 6, and 7. One major QTL, Xap.Pp.CO-4.1 on LG4 was associated with Xap resistance in leaf, and two major QTLs, Xap.Pp.CO-1.2 and Xap.Pp.CO-6.1 on LG1 and 6, respectively, were associated with Xap resistance in fruit. In addition, one major QTL, Xap.Pp.CO-5.1 on LG5, was associated with Xap resistance on both leaf and fruit. Breeding for bacterial spot resistance in peach using marker assisted selection will be discussed.