Vitis Species from the Southwestern United States Vary in Their Susceptibility to Powdery Mildew

The European grapevine ( L.) has been cultivated in North America for about 500 years. One of the major limitations to its culture is the powdery mildew (PM) fungus, Schw. This study reports on the most extensive screening of species from the southwestern United States and northern Mexico for resistance to PM, testing 147 accessions of 13 species. In addition, cv. Carignane, a highly susceptible wine grape cultivar, was used as a reference to evaluate the effect of the inoculum 14 days postinoculation. Inoculation was done with a vacuum-operated settling tower using a broadly virulent isolate of , the C-strain. Resistant accessions (nine), moderately susceptible accessions (39), and highly susceptible accessions (99) were detected. The resistant accessions were then inoculated with an additional fungal isolate, e1-101, and they retained their resistance. species susceptibility was not associated with a North-South gradation, but Western species were more susceptible than Midwestern and Eastern species. All five of the accessions were susceptible, as were the accessions of . The species , , , and × had significantly more resistant to moderately susceptible accessions compared with , , , × , , , and , which had relatively more susceptible accessions than the other species. This research identified new sources of PM resistance in from the southwestern United States that could be incorporated into PM resistance breeding programs throughout the world.