Pierce’s Disease Epidemiology and Management

This continuing project studied what happens to the Pierce’s disease (PD) bacterium {Xylella fastidiosa) in various plant species. We selected plants that are preferred by the principal insect vector of PD in coastal California, the blue-green sharpshooter (BGSS). We used infective BGSSs to inoculate plants in the lab with X. Fastidiosa. After keeping the plants in a greenhouse or waiting for mechanically inoculated plants in the field to develop infections, we attempted to isolate the bacterium from the plants. We recovered X. Fastidiosa for both types of inoculation from buckeye, valley oak, elderberry, and big leaf maple. We recovered bacteria from ash, coast live oak and bay laurel from BGSS-inoculated plants in the greenhouse but not from field-inoculated plants. We did not recover bacteria from alder, black walnut, arroyo willow, red willow, or sandbar willow that had been inoculated by either method or from cottonwood that had been mechanically inoculated. We recovered X. fastidiosa from mechanically-inoculated wild plum, which was not tested using BGSS inoculation. We kept all greenhouse-infected plants outdoors at Oakville during winter, 1995-96 and returned them to the greenhouse to hold for retesting (culture) later this summer. Many normally deciduous species (oaks, for example) kept their leaves throughout the very mild winter. We have reisolated X. fastidiosa from overwintered California blackberry. An analysis of DNA differences among 29 strains from grape or almond grouped strains into 3 clusters: north coast strains (no almond tested), most almond strains (Central Valley), and southern and central California strains (almond and grape).