Functional Allele Validation for Bacterial Spot [Xanthomonas arboricola pv. pruni (Xap)] Fruit Resistance in Peach [Prunus persica (L.) Batsch]

Bacterial spot [Xanthomonas arboricola pv. pruni (Xap)] is a serious disease of Prunus spp. 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 in a low to medium disease pressure year and not effective at all in a high disease pressure year. Incorporating bacterial spot resistance in newly developed peach cultivars is a more promising control measure and has been a key trait of interest in peach breeding programs. Eight functional alleles, four at each QTL (Xap1 and Xap6, R1, R2, I and S respectively) associated with Xap resistance in peach fruit, have been identified in the RosBREED peach Crop Reference Set (CRS). The University of Arkansas breeding material comprises all detected  Xap alleles and this program has never sprayed to control the disease in the 50-year effort. Thus selections against Xap have been possible particularly in the warm and humid environment in Arkansas, which favors Xap infection and disease development. The goal of this study was to provide evidence of phenotypic performance for all alleles at Xap1 and Xap6 loci. Preliminary results from 2013 show that individuals  homozygous for susceptible alleles (S¹:S¹ and S⁶:S⁶) at both loci were significantly more susceptibility to fruit Xap in comparison to homozygous resistant individuals (R¹:R¹ and R⁶:R⁶)  (p<.0001*, Tukey HSD). Significant differences were not observed when comparing individuals homozygous for intermediate alleles (I¹:I¹ and I⁶: I⁶) with homozygous resistant (R¹:R¹ and R⁶:R⁶) and homozygous susceptible individuals (S¹:S¹ and S⁶:S⁶). A second year of phenotypic evaluation in 2014 of the same individuals and additional 2010 seedling populations, including field and controlled inoculations, will lead to further validation of all alleles at Xap1 and Xap6 loci. Additionally the SNP haplotypes at both loci are being converted into breeder friendly DNA tests for widespread adoption of marker-assisted breeding (MAB) for bacterial fruit spot resistance.