Homalodisca coagulata (Say) is a sharpshooter leafhopper vector of the bacterial plant pathogen Xylella fastidiosa. Introduced into California about 15 years ago, this insect triggered recent outbreaks of Pierce's disease of grapevine in the state. H. coagulata has been observed feeding on dormant grapevines during the winter, raising the possibility of X. fastidiosa transmission during that season. We tested whether H. coagulata can acquire X. fastidiosa from and inoculate the bacterium to dormant grape (Vitis vinifera) in the laboratory and in the field. Usually, > 90% of H. coagulata survived on dormant plants in the laboratory and field. Field experiments showed that H. coagulata can inoculate X. fastidiosa into dormant plants, yet field acquisition experiments did not result in transmission. Transmission to dormant plants during the winter is a potential problem in California vineyards adjacent to citrus groves or other habitats with large overwintering populations of H. coagulata. Because dormant plants have positive root pressure, our findings provide evidence that K. fastidiosa transmission does not require negative pressure in plant xylem to be inoculated into plants.