Pierce’s Disease Epidemiology and Management

This is the final year of a continuing project on the fate of the Pierce’s disease (PD) bacterium (Xylella fastidiosa) in various plant species. The selected plants are preferred by the principal insect vector of PD in coastal California, the blue-green sharpshooter (BGSS) or are common perennials in riparian vegetation. We inoculated plants in the lab with X. fastidiosa using infective BGSSs and inoculated plants in the field mechanically with cultured bacteria. After keeping the plants in a greenhouse or waiting 2-4 months for mechanically inoculated plants in the field to develop infections, we attempted to isolate the bacterium from the plants. We previously were not able to recover by culture X. fastidiosa from arroyo willow, red willow, sandbar willow, or yellow willow that had been inoculated by either method, but polymerase chain reaction assays indicated the presence of X. fastidiosa in willows in the field. After inoculating red or yellow willows with X. fastidiosa using BGSS, we recovered the bacterium in culture after less than 1 week (8 of 15 positive) and after 3 weeks (8 of 13 positive) but made only one recovery in 16 attempts after 12 weeks. We concluded that the bacterium undergoes a short period of growth in willow, but does not survive well beyond 2-3 months. This would explain previous reports that willow is a host of the bacterium, and might have caused the positive PCR results in willows from which we did not culture X. fastidiosa. To examine the effects of overwintering, we are maintaining greenhouse-infected plants outdoors at Oakville during the winter of 1996-97. These will be returned to the greenhouse in March, 1997 to hold for retesting (culture) later this spring. We have further refined and expanded our analysis of strains of X. fastidiosa. Pierce’s disease strains from Georgia and Florida are closely related to California strains, especially north coastal California strains. Strains from peach (phony disease), oak leaf scorch, and oleander leaf scorch are all quite distinct from grape and almond strains. Two of the almond strains overlapped slightly with grape strains.