Use of Molecular Markers to Correct Grape Breeding Errors and Determine the Identity of Novel Sources of Resistance to Xiphinema index and Pierce’s Disease

Over the past 15 years, the grape breeding program at the University of California, Davis, has been evaluating Vitis rupestris x Muscadinia rotundifolia selections for resistance to the dagger nematode, Xiphinema index, and to Pierce’s disease (PD). Selections from these crosses exhibit very strong resistance to X. index and PD. In addition to breeding efforts, populations from these crosses have been used to develop genetic maps and locate resistance loci. Genetic mapping efforts recently began incorporating SSR markers to refine and expand existing maps. The use of SSR markers revealed that the mapping population parents were not crosses of V. rupestris x M. rotundifolia. This discovery led to testing of the entire group of 161 V. rupestris x M. rotundifolia progeny. All possible male parents surrounding the V. rupestris female parents in the vineyard where the crosses were made were genetically fingerprinted with up to 15 SSR markers to determine the true male parents. Results indicated that most of the male parents were from collections of forms of V. arizonica gathered in Mexico in 1961. These now correctly identified selections represent novel sources of very strong resistance to X. index and PD.