Grapevine Susceptibility to Pierce's Disease I: Relevance of Hydraulic Architecture

Abstract

The arrangement of vascular tissue within the nodes of Chardonnay grapevine (Vitis vinifera) shoots was studied as an investigation of potential pathways of infection for the bacterium Xylella fastidiosa, the cause of Pierce's disease. Grapevine stem anatomy of the current year's growth was observed with both light and scanning electron microscopy, and xylem conductance was observed by following the movement of stains within the xylem. Four to eight leaf traces diverge from the stele, which may or may not fuse to form a reduced number, and anastomosis of these traces begins before entering the petiole proper. No evidence was observed to support the report that leaf traces are distinct for four nodes prior to divergence and leaf traces are not considered to be conductively isolated. Lateral divergence of stem xylem into tendrils and lateral shoots creates a repeating pattern of gaps in the node that may preclude long-distance movement of bacteria in these sectors of the stem. The hydraulic architecture of the grapevine node is described and implications for the spread of Pierce's disease within the grapevine shoot are discussed.