Grapevine susceptibility to Pierce's disease II: Progression of anatomical symptoms
The progression of anatomical symptoms of Pierce's disease in susceptible Vitis vinifera cv. Chardonnay grapevines was studied over a time course following controlled inoculation of Y-shaped, two-shooted plants with Xylella fastidiosa. Leaf, petiole, and stem tissue samples collected from locations on inoculated and noninoculated shoots were taken at 10, 14, and 18 weeks following inoculation. After 10 weeks, leaf necrosis was visible in the inoculated shoots. Leaf and petiole xylem was predominantly occluded with gums and bacteria, whereas stem xylem was occluded almost exclusively with tyloses. After 14 weeks, bacteria and gums were found in noninoculated shoots, but tyloses were absent from these stem tissues. At 18 weeks, tylose development was observed in the xylem in stems of noninoculated shoots. Consequently, basipetal movement of bacteria occurred from the inoculated shoot into a noninoculated shoot. Results indicate that there is a predictable initial sequence of symptoms within infected shoots: gum and bacterial accumulation in leaves and petioles followed by tylose formation in stem tissue. This pattern is repeated with colonization of new shoots, eventually creating a systemic presence of the disease.