Evaluating Bacterial Spot [Xanthomonas arboricola pv. Pruni (Xap)] Resistance for Molecular Characterization and Incorporation of Marker-assisted Breeding (MAB) into the University of Arkansas Peach and Nectarine Breeding Program

Bacterial spot [Xanthomonas arboricola pv. pruni (Xap)] is a serious disease of Prunus spp. worldwide that causes premature defoliation, weak vigor of the plant, unmarketable fruit, and decline in production. Effective control methods are lacking; anti-bacterial sprays (copper-based compounds and oxytetracycline) are only partially effective. Incorporating bacterial spot resistance into newly developed peach cultivars is a more promising control measure and has been a key trait of interest in the University of Arkansas (UA) program. Marker-assisted breeding (MAB) is being pursued in the UA program to enhance traditional breeding techniques. Phenotypic data for Xap was collected in 2013 and 2014 across a range of cultivars, selections, and 13 seedling populations, using both field and laboratory data. Previously identified quantitative trait loci (QTL) for Xap fruit resistance (G1XapF and G6XapF) were analyzed across the material. The resulting alleles at each SNP loci were associated with the phenotypic data to differentiate individuals with susceptible (su) and or resistant (R) alleles. The validated SNP loci were directly used in marker-assisted parent selection (MAPS) in 2014, to pool resistance (R) alleles into the progeny. The SNP loci were next converted into breeder-friendly PCR-based DNA tests and used in marker-assisted seedling selection (MASS) in 2015 on several peach crosses to cull seedlings with su alleles. Pedigree-Based QTL Analysis (PBA) and association analysis are also being performed to identify additional QTLs controlling Xap resistance. Promising loci are being validated across the UA program and will be incorporated into MAPS and MASS in 2016 to enable further pyramiding of R alleles into high-quality cultivars.