XYLEM CHEMISTRY MEDIATION OF RESISTANCE TO PIERCE’S DISEASE Project Leader:

INTRODUCTION Xylella fastidiosa (Xf) is a Gram-negative xylem-limited bacterium that causes Pierce’s disease (PD), plum leaf scald, almond leaf scorch, phony peach disease and many other diseases. For many plant species there is no resistant germplasm. Xylem vessels may be filled with exopolysaccharides produced by the bacterium, and pectins, gums and tyloses produced by the plant. There has been considerable interest in the production of exopolysaccharides as a component of disease symptomatology, whereas the formation of gums and tyloses has sometimes been considered an adaptive host plant response to infection (Fry and Milholland 1990). The resultant plant/bacterium interaction between bacterium and plants results in xylem dysfunction, water stress and leaf necrosis, which are characteristic of PD.