THE EPIDEMIOLOGY OF PIERCES DISEASE

• Author(s): Hashim, Jennifer; Hill, Barry; Purcell, Alexander;
• Abstract: The epidemiology of Pierces disease (PD) changed dramatically in California with the arrival of the glassy-winged sharpshooter (GWSS) about 15 years ago. Before that time infections that persisted and resulted in vine death were the result of primary spread, i.e. from inoculum sources outside the vineyard. The disease caused losses, but the spread was linear, not logarithmic, and the damage was a gradual linear accumulation resulting in the loss of a small percentage of vines. With the exception of some traditional hotspot areas, losses from PD were important but not severe enough to preclude grape production. With the arrival of the GWSS, however, the transmission of the causal bacterium appears to be both primary and secondary (from vine to vine) and subsequent disease spread has become logarithmic, such that entire vineyards can be destroyed in as little as 3 to 5 years (Perring et al. 2001; Blua, Phillips et al. 1999; Purcell and Saunders 1999). To cope with this development there have been extensive field studies to determine methods to control the glassy-winged sharpshooter. However characterization of the changes in the epidemiology of PD when the causal bacterium is transmitted by GWSS has been based largely on anecdotal information and general observations with limited actual field data. These two coordinated projects propose to use field data from large numbers of vineyards to assess the impact of the glassy-winged sharpshooter on the epidemiology of Pierces disease. This is the first year of a proposed 5 year project. The resulting improved understanding of PD epidemiology may also enable UC Cooperative Extension to propose some preliminary recommendations for disease-based control strategies that growers can implement. Two critical issues are how much economic loss can be expected where GWSS occurs or when the insect moves into new viticulture areas, and what disease-based control methods can be employed in areas already infested with GWSS. The current economic loss models for GWSS are not based on empirical data but on arbitrary projections. Empirical mapping and disease tracking data that enables the comparison of various epidemiological factors (such as cultivar and susceptibility, vineyard age, proximity to GWSS hosts, cultural and control practices in grapes and other crops, etc.) are needed to make better informed projections. Current epidemiological models based on other native sharpshooter vectors (Purcell 1981) are not adequate to account for vine-to-vine spread when GWSS is the vector. Historically, mapping the incidence and vine locations of PD and tracking the spread over a few consecutive years has led to key conclusions regarding the sources of PD spread (Hewitt and Houston 1941, Purcell 1974) and the effectiveness of various control methods (Purcell 1979, Hewitt, Frazier et al. 1949). For example, these previous efforts paid off in identifying the highest risk areas to be avoided with new grape plantings.
• Publication Date: Dec 2002
• Journal: 2002 Pierce's Disease Research Symposium