Seasonality drives Pierce's disease epidemiology in California vineyards

In grapes the bacterium Xylella fastidiosa causes Pierce's disease (PD), resulting in leaf scorch symptoms and vine death. Xylella is vectored by many species of xylem-feeding insects, the most important of which are leafhoppers. One of these, the glassy-winged sharpshooter (GWSS; Homalodisca coagulata), is native to the Southeastern U.S but has become established in California within the last 15 years. Since GWSS's establishment, devastating PD outbreaks have occurred in California vineyards. Therefore GWSS has become the primary focus of research into the ecological conditions that promote PD outbreaks. The most common explanation for GWSS's importance is that, unlike native sharpshooters, GWSS is active and can transmit Xylella to vines through-out the year (even woody dormant vines), increasing the window for vine-to-vine spread. However, many of these secondary infections do not persist over winter, especially those occurring later in the season. Moreover, vector acquisition rates of Xylella are expected to vary seasonally. The implications of these seasonal factors for PD prevalence are not well understood. We used a vectored SI disease simulation model to clarify the consequences of seasonal vine recovery and vector acquisition on PD epidemiology. The results suggest that low acquisition rates early in the season and high recovery rates of late season infections temper disease outbreaks. However, high rates of pathogen retention (within the vector) over winter and, especially, high vector densities rapidly swamp out the ameliorating effects of seasonality. These results are relevant for evaluating the effectiveness management techniques, such as rouging, and for pinpointing the windows of vulnerability to chronic secondary disease spread.