An American hybrid vineyard in the Texas Gulf Coast: Analysis within a Pierce's disease hot zone
The incidence of Pierce's disease (PD) within an American hybrid vineyard in the Texas Gulf Coast was evaluated. Multiple viticultural parameters and Xylella fastidiosa levels were assessed for the varieties Black Spanish, Blanc du Bois, and Cynthiana in this region of intense disease pressure. Potential PD symptoms were observed uniformly in Cynthiana and sporadically in Blanc du Bois and Black Spanish postharvest. Consistent with these symptoms, Cynthiana showed significantly lower crop yield than either Blanc du Bois or Black Spanish and had significantly higher positive ELISA values for X. fastidiosa in summer petioles. Individual Cynthiana vines with higher bacterial levels as determined by ELISA exhibited correspondingly lower yields (p = 0.06). Indirect immunofluorescence microscopy of petiole xylem sap and PCR of petiole DNA confirmed positive X. fastidiosa ELISA readings in all three varieties. Cynthiana petioles tested by ELISA showed a dramatic increase in bacterial levels over the growing season. In contrast, bacterial levels in Black Spanish and Blanc du Bois remained lower all season despite high disease pressure. An evaluation of potential local plant reservoirs for X. fastidiosa demonstrated that at least six asymptomatic plant species tested positive by both ELISA and PCR: flex vomitoria, Vitis mustangensis, Rubus trivialis, Platanus occidentalis, Lippia nodiflora, and Lonicera japonica. In addition, June insect traps contained a significant number of X. fastidiosa-positive glassy-winged sharpshooters, Homalodisca coagulata, and several positive sharpshooters identified as Oncometopia orbona, suggesting the presence of multiple species of insect vector in this region.