Grapevine cultivar susceptibility to Xylella fastidiosa does not predict vector transmission success

For vector-borne diseases, interactions among vector, host and pathogen can influence patterns of disease spread. In particular, previous studies suggest host genotype may influence disease dynamics due to differences in susceptibility to the pathogen and, therefore, subsequent vector transmission efficiency from these plants. We tested this hypothesis for the pathogenic bacterium Xylella fastidiosa, the etiological agent of Pierce’s disease in grapevines, and its leafhopper vector Homalodisca vitripennis. We compared pathogen infection level and transmission efficiency among several widely cultivated red and white wine, table and raisin grape cultivars, with the expectation that vector transmission rate would differ among cultivars because of underlying differences in susceptibility to infection. The 14 grapevine genotypes evaluated showed significant differences among cultivars in the populations of X. fastidiosa that developed in petioles. Flame seedless hosted the highest bacterial populations in its petioles, between 2.05 to 1.81 times higher than the least susceptible cultivars Rubired, Grenache Noir, Crimson Seedless, and Merlot. However, although the transmission rate of X. fastidiosa by H. vitripennis varied substantially (zero to 33%), it was not significantly different among cultivars. These results suggest that either the relationship between vine infection level and transmission are weaker than previously reported or innate differences in vector preference among cultivars confounded any effects of vine susceptibility to infection.