Pierce’s disease (PD) limits cultivation of European grape (Vitis vinifera) in the US. The disease is caused by the xylem-limited bacterium Xylella fastidiosa. Genetic resistance to the disease is found in some native Vitis, and such species have been incorporated into breeding programs that attempt to combine PD-resistance from native Vitis with fruiting and flavor characteristics of V. vinifera . A single dominant gene (PdR1) from Vitis arizonica was backcrossed into V. vinifera table and wine grape cultivars. In an effort to develop a rapid and repeatable screening assay for evaluating the progeny of the backcross, we determined the relationship between populations of X. fastidiosa, as determined by ELISA, in resistant and susceptible genotypes. Our findings indicate that expression of PdR1, as defined by bacterial levels in stem tissue, is dependent on environmental factors. This study identifies some of the environmental factors that influence resistance to PD and suggests that the mechanism of resistance is sensitive to light and other sources of stress in the greenhouse environment.