The source of Pierce’s disease (PD) resistance in the Walker lab breeding program is derived from a form of Vitis arizonica that Harold Olmo collected in Monterrey, Mexico. This resistance is inherited as a single dominant gene named PdR1 and is located on chromosome 14. To identify other forms of resistance that could be stacked to produce more durably and broadly resistant winegrapes, we evaluated over 250 accessions from the southwestern United States and Mexico. These accessions were greenhouse-screened for resistance and genetically characterized with chloroplast and SSR markers. Greenhouse screening identified multiple accessions that supported very low levels of PD’s causal agent, Xylella fastidiosa. The genetic characterization identified five major groupings, within which 15 highly resistant accessions were grouped in clades different than those housing PdR1. Crosses were made among these resistant accessions and susceptible V. vinifera to develop small breeding populations that were greenhouse-screened to determine the mode of inheritance in the different resistant parents. To ensure the resistances differed from the PdR1 locus, a limited mapping strategy was used to scan the chromosome 14 region. Twelve SSR markers that flank the PdR1 locus and cover 3.5Mb of physical distance were used to genotype 704 seedling plants. Data from each population was analyzed to determine the distribution of resistance. Each marker was tested for deviation from normal segregation and haplotypes were constructed. The linkage data was used to carry out QTL analysis for each population. The resistant accessions will be discussed in terms of their genetic differences and potential to provide unique and strong sources of PD resistance for breeding.

Funding Support: CDFA Pierce’s Disease Board, and from the Louis P. Martini Endowed Chair in Viticulture, the Louise Rossi Endowed Chair in Viticulture, and support for Karla Huerta from CONAYCT, Mexico