69 Bacterial Spot Resistance in Peach: Functional Allele Distribution in Breeding Germplasm K. Gasic and G. Reighard SAFES, Clemson University Clemson, SC 29634 USA W. Okie S.E. Fruit and Tree Nut Research Lab. USDA-ARS, Byron, GA 31008 USA J. Clark Department of Horticulture University of Arkansas Fayetteville, AR 72701 USA T. Gradziel Department of Plant Sciences University of California Davis Davis, CA 95616 USA D. Byrne Department of Horticultural Sciences Texas A&M University College Station, TX 77843 USA C. Peace Department of Horticulture Washington State University Pullman, WA 99164 USA T. Stegmeir, U. Rosyara and A. Iezzoni Department of Horticulture, East Lansing Michigan State University, MI 48824 USA Keywords: fruit, haplotype, RosBREED, QTL, Xanthomonas Abstract Bacterial spot, caused by Xanthomonas arboricola pv. pruni, is a serious economic disease causing severe defoliation and black surface pitting, cracking or blemishes on peach fruit worldwide. A management option for control and meeting consumer demand for chemical-free, environmentally friendly fruit production is the development of resistant or tolerant cultivars. Resistance to bacterial spot in peach cultivars is apparently under polygenic control. Several controlling loci in the peach genome conferring quantitative resistance were recently discovered in an F 2 population. The two quantitative trait loci (QTLs) with the largest effects on bacterial spot resistance in peach fruit, Xap.Pp.OC-1.2 and Xap.Pp.OC-6.1, were validated here in a large collection of U.S. peach breeding germplasm (Prunus persica and some closely related species). High-resolution genome scans of this germplasm conducted within the RosBREED project were associated with phenotypic data on fruit bacterial spot resistance to determine effects and distributions of functional alleles. This resulting predictive DNA information has immediate application in U.S. peach breeding. INTRODUCTION Bacterial spot, caused by Xanthomonas arboricola pv. pruni (Xap), is a serious disease that in favorable years can affect peach fruit quality and production in areas of the world where rainfall occurs during the growing season (Werner et al., 1986; Ritchie, 1995). Severe infection causes leaf defoliation, tree weakening and reduced fruit quality to the point of being unmarketable (Ritchie, 1995). Limited anti-bacterial spray options and an environmentally conscious public have resulted in renewed interest in breeding peach cultivars that are tolerant or preferably outright resistant. Previous research revealed a quantitative nature of resistance to bacterial spot in Prunus (Socquet-Juglard et al., 2012; Yang et al., 2013). Current selection processes based on phenotypic evaluation of fruit and specific environmental conditions eliciting disease development have prevented bacterial spot resistance from being efficiently bred into most modern peach cultivars. Two major Proc. VIII th Intl. Peach Symposium Eds.: C. Xiloyannis et al. Acta Hort. 1084, ISHS 2015