The work in this research project concerns three things. First, it is intended to improve understanding of what controls the spread of leafroll disease within and between vineyard blocks. Second, it aims to work out costs for finding and dealing with leafroll infections in California vineyards so that growers can make better-informed choices about disease management. Lastly, it is intended to look at some of the difficult issues concerning cooperation and shared costs and impacts in managing leafroll at a neighborhood level, and to act as a focus for outreach from UC Davis to support the grower community and UC Cooperative Extension in tackling leafroll disease.
Our analysis of leafroll disease progress data shows that the disease develops in a predictable way irrespective of grape variety. The disease is typically introduced to healthy vine blocks at random locations, consistent with dispersal of mealybug juveniles in wind gusts. Spread between infected and healthy blocks may cause these initial infections to edges of healthy blocks, but random infections, well away from the edges, are also possible. Random initial infections could also arise, in theory, from infected planting material, but cases where this happens would be expected to show up one to two years after block establishment or vine replacement and so should be identifiable by reference to block age when disease first appears. Once introduced to a block, disease intensifies around the initial infection in a way that is consistent with mostly plant-to-plant spread of mealybug crawlers.
The research on epidemic dynamics feeds into our second area of work. As part of the epidemiology studies we have characterized the degree of clumping of diseased vines around the initial infections. This statistical analysis of the pattern of diseased vines allows us to calculate the effect of clumping on sampling efficiency for detecting the disease. That is, we can work out how the tendency for diseased vines to occur in small focused patches initially affects the efficiency of time spent sampling for disease and also on the accuracy of estimates of the level of disease. In general, the level of patchiness we find for leafroll has significant impacts on both the efficiency of sampling and the certainty of estimates based on sampling. We provide some illustrative results from this analysis.Neighborhood groups for managing leafroll have now been established in the Napa region, partly in response to suggestions made in the early stages of this project. We have extended the work reported last year on attitudes among growers to include representatives of the grapevine nursery industry. The results show that individuals from nursery trade have a similar range of attitudes towards leafroll as growers. There was some evidence that different nursery companies may have a recognizable company-level collective attitude, but the sample size is small. Our modeling work of disease dynamics at the neighborhood scale has highlighted the importance of disease management within existing infected blocks. The contribution of new infections from infected planting material is relatively small when there is a high background level of disease from existing infections.