CHARACTERIZATION OF RESISTANCE TO PIERCE’S DISEASE IN MUSCADINIA ROTUNDIFOLIA AND THE POTENTIAL FOR CONFERRING RESISTANCE TO GRAFTED VITIS VINIFERA SCIONS Project Leaders:

Muscadinia rotundifolia is a wild grape native to the Southeast U.S. In field trials, as well as in the wild, this species exhibits strong resistance to Pierce’s disease (PD). Incorporating this resistance into Vitis vinifera production systems in California will require identifying prime breeding material and increasing our understanding of the mechanism of resistance. The first objective of this study was to determine if resistance to Xylella fastidiosa (Xf) and PD varies among wild M. rotundifolia populations across a natural gradient of disease pressure. Greenhouse trials are underway using material collected in Florida, Georgia and Tennessee. The second objective was to test the common belief that some M. rotundifolia cultivars are more Xf resistant than others are. Again, trials are currently underway. Thirdly, we want to test if Xf resistance in M. rotundifolia is consistent when challenged with different Xf strains from Florida and California. Lastly, we want to examine the potential for conferring Xf and PD resistance to V. vinifera scions by grafting them to resistant M. rotundifolia rootstocks. This project is ongoing and only very preliminary results are available now. One such result is that even the least resistant M. rotundifolia selections are much more resistant than the Vitis vinifera cultivars. We have also confirmed that supposed graft incompatibility between the two species can be overcome using green grafting techniques. These grafted plants are entering disease trials now. INTRODUCTION Use of resistant plant material is a proven approach to solving crop pest and disease problems. This approach can provide a robust solution that, among other advantages, avoids the potential negative environmental impacts associated with a chemical control strategy. Unfortunately, in the case of Pierce’s disease, it appears that all the common cultivars of V. vinifera are susceptible to the disease (Raju and Goheen 1981). Historically, breeders and researchers have looked to the wild grape species of the southeastern U.S. for potential sources of PD resistance (Loomis 1958, Mortensen et al. 1978). PD pressure is very high in this region and many of the local species are indeed resistant or tolerant. In particular, M. rotundifolia appears to have exceptional resistance and is planted extensively in Florida (Loomis 1958, Hopkins et al. 1974). Efforts have been made to create PD resistant bunch grapes by crossing M. rotundifolia with V. vinifera. Initial success was limited because of genetic incompatibility, but efforts continue and some fertile hybrids are available (Lu et al. 2000). Such hybrids are also being used in work aimed at identifying the genes responsible for the resistance (Krivanek and Walker 1999, 2000). The current project is aimed at increasing our understanding of how PD resistance (or tolerance) functions in M. rotundifolia with the thought that such knowledge will improve our chances of incorporating resistance into V. vinifera. We will also explore the potential for conferring this resistance to V. vinifera via grafting. OBJECTIVES 1. Determine if resistance to Xylella fastidiosa (Xf) and Pierce’s disease varies among wild M. rotundifolia populations across a natural north/south gradient of disease pressure. 2. Test the common belief that some M. rotundifolia cultivars are more Xf resistant than others. 3. Determine if Xf resistance in M. rotundifolia is consistent when challenged with different Xf strains from Florida and California. 4. Determine if there is any potential for conferring Xf and PD resistance to V. vinifera scions by grafting them to resistant M. rotundifolia rootstocks. RESULT Variation in resistance in wild M. rotundifolia We completed a collection trip in June 2003 that covered the range of PD pressure from Florida (high) to Tennessee (low). Replicated potted cuttings from 11 collection sites are now in a resistance trial in the greenhouse (n = 138, including controls). This experiment serves as an indirect test of the hypothesis that Xf resistance in M. rotundifolia reflects an evolved response to the disease. Results indicating that resistance tracks disease pressure would suggest that resistance did evolve in response to the disease. We would therefore expect the mechanism to be something specific such as an induced defense that results from some chemical elicitor. If resistance does not appear to be an evolutionary response, we might expect a more general mechanism such as basic differences in xylem structure or xylem sap composition.