BIOLOGICAL, CULTURAL, GENETIC, AND CHEMICAL CONTROL OF PIERCE'S DISEASE: SIGNIFICANCE OF RIPARIAN PLANTS IN THE EPIDEMIOLOGY OF PIERCE’S DISEASE Project Leader:

The goal of this research is to evaluate the significance of riparian hosts in the epidemiology of Pierce’s disease (PD) in the North Coast grape-growing region of California. Our first objective is to examine the epidemiological role of seasonal Xylella fastidiosa (Xf) concentration fluctuations in riparian hosts in the field, where plants are subject to seasonal temperature changes. Among systemic riparian hosts, differences in seasonal Xf concentrations and Graphocephala atropunctata (blue-green sharpshooter, BGSS) feeding preference affect their importance as Xf reservoirs. Temperature affects Xf concentrations in plant hosts and, in turn, Xf concentrations affect the probability of a BGSS acquiring Xf while feeding on an infected plant. We focused on Xf concentrations in five systemic hosts: Rubus discolor (Himalayan blackberry), R. ursinus (California blackberry), Sambucus mexicana (blue elderberry), Vinca major (periwinkle), and Vitis californica (California grapevine). We needle inoculated potted plants of California grape, California blackberry, Himalayan blackberry, blue elderberry, and periwinkle in the greenhouse. After confirming infection with PCR, we transferred infected plants to two sites in the North Coast (Napa County and Mendocino County). Populations of Xf reached detectable levels in all five riparian hosts located at our Napa County site in October 2003. Every replicate plant of periwinkle and California grapevine showed typical leaf scorch symptoms of PD, which is not surprising given the high concentrations of Xf detected among them. The fact that none of the Himalayan blackberry showed symptoms, despite high Xf concentrations, suggests that Himalayan blackberry is more tolerant of Xf infection. Assuming our results reflect that of naturally-established riparian hosts in the field, Xf concentrations in California grapevine, Himalayan blackberry, and periwinkle are still sufficient for acquisition by BGSS in autumn. Dilution plating results have not yet been obtained from our Mendocino County site and real-time PCR reactions have not been completed for plants from either site. We will continue to estimate Xf concentrations in plants at both sites using dilution plating and real-time PCR on a seasonal basis. Xf concentration estimates from each plant species at each site will be averaged among replicate plants. The effects of plant species, season, and location on mean Xf concentration will be determined using an analysis of variance. Results obtained using the two quantitation techniques will be analyzed separately. INTRODUCTION Past research (Purcell 1976, 1981) demonstrated the direct relationship between incidence of Pierce's Disease (PD) in Vitis vinifera and proximity to riparian plants bordering vineyards in the North Coastal grape-growing region of California. Vineyard rows closest to riparian plants that occupy the banks of rivers and streams experience the heaviest losses, but rows have fewer diseased vines with increasing distance away from riparian plants. Riparian habitat adjacent to vineyards contains plant species that serve as feeding and breeding hosts for Graphocephala atropunctata (blue-green sharpshooter, BGSS), the most efficient vector of PD in the Napa Valley (Hewitt et al 1949, Purcell 1975). Not only do many riparian plant species provide habitat for BGSS, but some also serve as reservoir hosts of the PD strain of Xylella fastidiosa (Xf) (Freitag 1951). A variety of common riparian plants, including native and non-native trees, shrubs, and herbaceous annuals, are capable of maintaining Xf infections without expressing disease symptoms. Purcell and Saunders (1999) found that Xf populations are, generally, lower in riparian hosts than in grapevines. The ability of Xf to multiply and spread within a plant host varies from species to species. After screening several breeding hosts of BGSS for systemic movement of Xf, Hill and Purcell (1995) found that only two, Rubus discolor (Himalayan blackberry) and Vitis vinifera (grapevine), supported systemic Xf populations. These results imply that some riparian hosts are likely more important than others as reservoirs for the spread of Xf to grapevines. Interactions among BGSS, Xf, and their host plants are likely to vary from season to season. Aside from the obvious effects of season on BGSS breeding, seasonal changes in BGSS flight activity have been documented (Feil et al 2000). Seasonally variable levels of plant hormones (Hopkins 1985) and changes in temperature (Feil and Purcell 2001) can have major effects on Xf concentrations in host plants. Xf concentrations change on a seasonal basis in Vitis labrusca (Hopkins and Thompson 1984) and they are lower in V. vinifera grown at cooler temperatures (Feil and Purcell 2001). The efficiency of Xf acquisition and transmission by BGSS is influenced by the concentration of Xf in the plant host; the higher the concentration of Xf, the higher the probability of BGSS acquiring Xf while feeding (Hill & Purcell, 1997). Therefore, we might expect that in