This project continues to develop management strategies for grape growers with grapevine leafroll disease. We recently completed a mapping project spanning 4 grape growing seasons (2010-2013) to determine the relationship between grape mealybug (GMB) populations and the incidence of grapevine leafroll disease (GLD) in Napa County vineyards. We surveyed 10 vineyards and conducted a preliminary analysis to demonstrate the link between the vector, GMB, and disease incidence. In vineyards with medium to high levels of GLD, rates of disease spread are influenced mainly by (1) the initial number of vines in a vineyard with GLD (disease pressure) and (2) GMB populations in the previous growing season. In vineyards with low levels of GLD, rates of disease spread are influenced by the initial disease incidence as well as the number of GMB found in the current season. These observations are consistent with similar studies in other grape growing regions around the world, and have clear implications for management: in order to successfully manage GLD incidence and spread, growers must evaluate disease pressure and GMB populations. Our preliminary analysis suggests that managing the vector alone may not be sufficient to decrease rates of disease spread, particularly in vineyards with medium to high GLD incidence.
Studies of other pests and diseases have demonstrated that a regional approach to disease management has more potential for success than individual efforts. This is especially true in Napa County, where a single or few growers rarely control large swaths of contiguous vineyard acreage. In Napa County, where neighboring vineyards share the burden of disease management, growers may jointly launch regional responses to GLD. We are developing a template for these regional efforts, with a pioneering group of 20 grape growers farming 1900 contiguous acres. The grower group is committed to implementation of coordinated, regional GLD management strategies that include monitoring of GLD and GMB, as well as lowering disease and vector pressure. Most importantly, the group shares information, communications and develops goals and activities at regular meetings (4-6 meetings per year) focusing on the implementation of GLD management strategies at a regional level.
This project is also developing the use of hyperspectral imaging to map the incidence of GLD in commercial vineyards and nurseries. Imaging spectroscopy provides a potentially valuable alternative to lab testing and field scouting in that it is efficient, non-destructive and relatively inexpensive. From an aircraft thousands of acres may be imaged in a single three-hour flight. Additionally, remote sensing provides continuous measurements to construct a map of the target vineyard. Our studies in Cabernet Sauvignon vineyards have shown a 94.7%correlation between leafroll-diseased vines identified in ground surveys with those identified through aerial imaging, suggesting a high level of confidence in the accuracy of our measurements for this variety, and potential to develop the use of this technology for other grape varieties. Hyperspectral airborne imaging could revolutionize the the collection of disease incidence data by increasing ease, accuracy and efficiency of measurements.