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Vitis Resistance to Pierce's Disease Is Characterized by Differential Xylella fastidiosa Populations in Stems and Leaves

The pattern of Xylella fastidiosa infection in resistant and susceptible grapevines representing a diverse selection of Vitis spp. was characterized through measurements of X. fastidiosa bacterial movement and accumulation in artificially inoculated greenhouse-grown grapevines. A double-antibody sandwich enzyme-linked immunosorbent assay (ELISA) was optimized for quantification of X. fastidiosa populations and tested on known amounts of X. fastidiosa added to grape tissue extracts. Predicted versus known X. fastidiosa concentrations proved to be highly correlated (R² = 0.99). Populations of X. fastidiosa in stem internode, stem node, petiole, and leaf blade samples from the genotypes in this study were measured at 12 weeks postinoculation using the optimized ELISA procedure. Samples from each plant part were taken at eight positions along the inoculated shoots. Systemic infection was detected in both susceptible and resistant genotypes. Resistant genotypes were characterized by significant differences in X. fastidiosa populations between stem internodes and leaves (1.0 × 10⁶ and 1.1 × 10⁷ cells/g of sample, respectively). In contrast, the susceptible genotypes were characterized by high mean X. fastidiosa populations in both stems and leaves (5.6 × 10⁷ and 4.8 × 10⁷ cells/g, respectively) the latter of which were not significantly different from the resistant genotypes. A high correlation (R² = 0.97) between stem X. fastidiosa numbers to previously characterized field Pierce's disease (PD) performance indicates that the quantitative ELISA measurements of X. fastidiosa in greenhouse-grown grapevines should be a useful tool for predicting PD resistance under field conditions.