2018 ASHS Annual Conference

Bacterial spot, caused by the bacterium Xanthomonas arboricola pv. pruni (Xap), is a threat to the plum (Prunus domestica L), cherry (P. avium L.), and peach (P. persica L.) industries. This disease causes premature defoliation, reduced vigor and productivity, and yield loss due to unmarketable fruit in peaches grown in humid regions around the world, including the Eastern U.S. Recommended cultural practices to control Xap include removal of infected tissue and application of bactericides containing a mix of copper and oxytetracycline. The development of bacterial spot resistant peach cultivars could help to mitigate the environmental and health risks of bactericides while reducing input costs for growers. Because Xap pressure varies from year to year depending on environmental conditions and disease incidence is low in some important breeding sites, molecular markers associated with bacterial spot resistance can help breeders to develop resistant cultivars efficiently. Markers for fruit resistance to bacterial spot have been developed from quantitative trait loci (QTL) discovered on linkage groups (LG) 1 and 6 as part of the RosBREED project and are currently used in the University of Arkansas System Division of Agriculture (UA) and Clemson University peach breeding programs. However, no markers associated with foliar resistance to bacterial spot have been developed. Seven populations and their parents (n = 210), were evaluated for fruit and foliar resistance in 2017 as part of a genomewide association study (GWAS). Phenotypic data included visual ratings of fruit and leaves infected with natural inoculum at the UA Fruit Research Station in Clarksville, AR and a detached leaf bioassay performed in the lab under sterile conditions. All individuals were genotyped with 16K peach genotyping array and GWAS was performed using a Q+K model. Foliar resistance was quantitatively inherited and controlled by several small-effect QTL located on chromosomes 1, 3, 4, 6, and 8, each explaining 5-15% of observed phenotypic variation. Only one QTL for fruit resistance was found on chromosome 5 in a position overlapping with the IndelG locus controlling fruit pubescence. Several of the populations evaluated contained both peaches and nectarines, and nectarines had more severe fruit symptoms on average. This experiment will be repeated for a second year during 2018.