GRAPEVINES AND PIERCE'S DISEASE: A XYLEM FLUID MINERAL-STATUS DEPENDENT CONDITION

Pierce’s disease (PD) of grapevine is caused by the pathogenic bacterium Xylella fastidiosa. PD precludes the establishment of high quality cultivars of Vitis vinifera in the southeastern United States. In addition, PD also caused millions of dollars in losses to the California grape and wine industry. PD is transmitted by leafhoppers, which feed on the nutritionally poor xylem fluid. The disease ultimately causes a xylem vessel malfunction resulting in plugging and the interruption of regular water and nutrients flow, resulting in leaf scorching. There are no resistant/tolerant varieties among V. vinifera, and there is no practical cure. X. fastidiosa may be found in the xylem vessels of numerous plant species, but only causes problems for a few. Xylem fluid is highly diluted and contain inorganic ions, amino acids and organic acids. Once bacteria are inoculated in a PD-susceptible plant they increase in population and establish a community of bacterial cells mixed with mucilaginous materials forming what is defined as biofilm (‘city of cells’). The growth of the bacterial population during the first steps of the biofilm formation is determined by the chemistry of the xylem fluid, especially by calcium (Ca) and magnesium (Mg) availability as free divalent ions. There is strong evidence that xylem chemistry of grapevines influences susceptibility to PD. In addition, the xylem fluid pH plays an important role as it regulates ions chemical state. Energy Dispersive Spectroscopy (EDS) data helped conceptualization of the “calcium bridging” as a way to explain cell aggregation and biofilm formation. Recently, PD lesions have been shown to accumulate higher concentrations of silicon (Si).This metabolic transition is possibly a result of the plant strategy to cope with xylem vessel occlusion due to bacterial multiplication and the reduction of water flow. Si accumulation in leaf scorched areas affected by PD is a unique strategy and might help to understand Si influences the concentration and availability of Ca and Mg. Since the xylem fluid chemistry is soil dependent, we can assume that soil chemistry is an important factor is the progress of PD.