This project continues to inform and support the adoption of management strategies to minimize the incidence of leafroll disease spread in California vineyards. During the 2014-15 project year, we completed several objectives: (1) we finalized our analyses of the factors impacting leafroll disease spread, (2) developed methods for hyperspectral airborne imaging to identify diseased vines, and (3) supported the efforts of a group of growers working on regional leafroll disease management strategies. Our analyses of factors affecting disease spread suggest that the most important determinant of new disease is “block”—that is, there is great variability in the incidence of new disease explained by factors that are specific to the block and vintage (management practices, locationparticularly in relation to other diseased blocks, variety, vine age, vine health, etc.) Disease pressure, defined as the number of diseased vines present in the vineyard in the prior year, also impacts the incidence of new disease. And, in vineyards with medium to high disease incidence, mealybug populations affect disease spread. This suggests that in vineyards with low levels of disease, management practices that support vine health and decrease the number of diseased vines are critical to minimize disease spread.
In vineyards with greater incidence of disease, management practices may also include strategies to minimize mealybug populations. These observations are consistent with similar studies in other grape growing regions around the world, and have clear implications for management: in order to determine which management strategies should be adopted, growers must evaluate disease pressure and GMB populations. In Napa County, where neighboring vineyards share the burden of disease management, growers may jointly launch regional responses to GLD. We have developed a template for these regional efforts, with a pioneering group of 20 grape growers farming 1900 contiguous acres. The grower group (LAMBA) 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. The group shares information and develops goals and activities at regular meetings and focuses on the implementation of GLD management strategies at a regional level.
We have also developed 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. Our preliminary analysis of hyperspectral images of Cabernet Sauvignon vineyards have shown 75 to 90%correlation between leafroll-diseased vines identified in ground surveys with those identified through aerial imaging. Further analyses planned for 2015 will refine these figures and help develop the use of this technology for this and other grape varieties.