Xylella fastidiosa are gram-negative bacteria that occur in the xylem of PD-infected grape plant, and are transmitted by xylem-feeding insects; leafhoppers and sharpshooters. Our hypothesis is that bacterial polysaccharides (EPS and LPS) produced by X. fastidiosa may be causative agents of Pierce’s disease on grapes.
We have chemically analyzed isolated PD-infected grape xylem sap and Xylella cultures for Xylella EPS and LPS. We have found no evidence for the production of a “xanthan-like” EPS gum. We concluded that the symptoms of Pierce’s disease are not due to a buildup of a Xylella ESP gum in infected vines. Physical occlusion of infected xylem tissue is therefore more probably due to adhering and clumping bacteria cells, and/or to the production of an abundant LPS.
Our research has identified a Xylella cell-wall LPS (lipopolysaccharide), composed of polymeric D-galactose and L-rhamnose sugars. Significantly, Xylella LPS from the grape strain is structurally different to LPS isolated from orleander-specific Xylella.
Perhaps most important, we have shown than an antibody raised against the Xylella grape-pathogen recognizes the purified LPS from this strain, but does not detect LPS from the orleander Xylella or from control bacteria. This antibody also detects the Xylella LPS in PD-infected grape sap isolated from plants in the field.
Our results suggested that an Xylella LPS antibody kit could be developed as a early diagnostic for Pierce’s disease infected vines, that could be used by the non-specialist in the field. With this goal in mind, we are presently developing a quantitative, color-based LPS-antibody dot blot assay for the early detection of PD.