Biological, Cultural, and Chemical Management of Pierce’s Disease

Studies on understanding how Xylella fastidiosa (Xf) moves in grape xylem were initiated using fluorescent antibodies and confocal microscopy. Xf bacteria were specifically labeled and identified in fixed grape tissues. A series of time course experiments to track Xf movement after insect or mechanical inoculation are underway. Field and greenhouse studies examining the effect of freezing temperatures on Xf survival are progressing. Pure cultures of Xf can be frozen without significant decreases in viability, however recovery of viable Xf from cold-treated canes was much lower than from non-treated canes. Experimental plots were established in several Napa vineyards to determine whether insecticide treated “trap crops” or vegetation barriers could reduce PD movement into vineyards. Vector abundance and movement is being monitored in these sites and PD incidence opposite tree barriers is being compared to that from sites without tree barriers. Four new sites were identified for the trap crop study, and St. George rootstock was planted in preparation of both foliar and soil applications of imidocloprid next spring. St. George has earlier budbreak than V. vinifera cultivars and may be used to trap early movements of sharpshooters. Extensive evaluations of this pesticide are being conducted to fine-tune its application rate and timing. In vitro Xf studies have established what concentrations of the micronutrients Zn, Cu, Mn, and Fe are toxic to Xf. Means of applying these micronutrients to grape are being evaluated, and the extraction xylem sap from grape canes has been optimized. An antibiotic produced by a Bacillus bacteria strain was shown to be very toxic to Xf on petri plates; studies for its use as a biocontrol agent for PD are initiated. Studies on the characterization of bacterial and fungal endophytes in grape xylem are also underway in vines from Davis and Napa. Work towards mapping resistance to Xf is progressing. Forty V. rupestris X M. rotundifolia selections have been tested and three have high levels of resistance (lack of symptoms and limited Xf movement based on IC-PCR results). Unfortunately, the mapping populations that were made previously, 8913-02 and 8913-21 crossed to Chardonnay, cannot be used because neither of these selections is adequately resistant. One of three highly resistant selections is 8909-15. A genetic map was created for Xiphinema index resistance using the 8909-15 X 8909-17 seedling population. This population may also be ideal to map Xf resistance since 8909-17 is susceptible. However, we also plan to produce other seedling mapping populations by crossing 8909-15 and the other resistant selections with Chardonnay. Crosses with Chardonnay will give a more clear range of responses from very resistant to very susceptible and should therefore be easier to map in. These crosses will also begin the introgression of Xf resistance fromM rotundifolia into V. vinifera. A vineyard of 10 Xf resistant / susceptible variety pairs is establishing in Florida were the effect of xylem chemistry on PD will be studied. A screenhouse was also completed so that V. vinifera varieties can be studied. Technicians and cooperators in Florida have been coordinating and testing equipment for xylem fluid analysis, Xf culture and PCR detection of Xf. Examinations into the possible role of peroxidases (known plant resistance compounds that exist at low concentrations in xylem) in Xf resistance are underway. Many recent resistance studies in other plants have identified peroxidases as important resistance compounds. Other organic and inorganic compounds, found at higher concentrations, will also be examined.