Resistance to Pierce's disease in Muscadinia rotundifolia and other native grape species
Pierce's disease (PD), caused by the bacterium Xylella fastidiosa, is a major disease of grapevines. Grape species native to southern areas of North America where the disease is severe have evolved resistance to PD. Using controlled greenhouse trials, a quantitative assessment of the level of resistance in cultivated and wild selections of five native grape species was made. Conclusions were based on estimates of bacterial concentrations in stem tissue via ELISA and subjective evaluations of disease symptoms. Vitis labrusca, native to the northeast United States where PD is absent, appears to be as susceptible as Vitis vinifera. California natives Vitis californica and Vitis girdiana appear to be moderately susceptible, although there was significant variation among the V girdiana selections. In contrast, Muscadinia rotundifolia and Vitis arizonica, both native to areas of severe disease pressure, appear to be very resistant. A pattern of resistance correlating with geographic variation in disease pressure was also evident within species. Wild accessions of M. rotundifolia from cooler areas such as Tennessee, where PD is uncommon, supported up to 20x higher concentrations of bacteria than the accessions from Florida where PD is severe. Trials with wild accessions of V girdiana showed a similar pattern, with susceptible selections supporting up to 100x higher bacterial concentrations. Results demonstrated a gradient of resistance both among and within species, consistent with the hypothesis that PD resistance has evolved in response to disease pressure. This study identifies candidate plant material for breeding projects and provides insight into the genetic and physiological basis of PD resistance in native grapes.