In grapevines, scion-rootstock grafting is a common practice to impart pathogen resistance and to manipulate aspects of grapevine physiology, including vigor, yield, and fruit composition. Successful grafting requires the integration of the scion and rootstock vascular networks. The nature and extent of this integration was evaluated to determine the impacts of the graft union on the movement of vascular pathogens between scion and rootstock.. Using both xylem-mobile dyes and the xylem-limited bacterial pathogen Xylella fastidiosa, we demonstrate that the graft union contains open xylem conduits providing for passive pathogen movement. These open conduits may facilitate bacteria overwintering in below-ground vine tissues of grafted vines and systemic infection in subsequent growing seasons.