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.

Grapevine leafroll-associated viruses (GLRaV) are a complex of viruses that cause leaf chlorosis and leaf margins to “roll” downward.  GLRaVs can reduce yields, delay fruit maturity, and impede fruit pigmentation. Our work focused on control measures for the insect vectors of GLRaV. In 2013 we monitored changes in vine mealybug (VMB) populations in three areas that were formerly part of an areawide mating disruption program and showed that populations have increased in two out of the three of the areas since the removal of pheromone dispensers. Trap catches surrounding historical “hotspots” were also elevated, suggesting that mealybug populations in these areas are spreading. We continued a six-year field study mapping the establishment and spread of GLRaV in a 20 acre vineyard planted in 2008 from certified virus-free scion and bordered by blocks with GLRaV-infected vines and mealybugs. PCR tests revealed that 17 out of 25 vines that were visually identified as symptomatic for GLRaV in 2011 and 2012 were in fact infected by red blotch, which was present in the vineyard by at least 2011. Red blotch-infected vines were randomly distributed inside the mapped plot.

We conducted a study of aerial grape and vine mealybug dispersal at two 10-acre vineyard sites in the Napa Valley, using sticky traps that were deployed in a grid pattern throughout each site. We have not observed mealybugs on the sticky traps that have been processed thus far, suggesting that aerial dispersal by mealybugs is uncommon in Napa vineyards. Work examining the temperature development rates of grape, obscure and Gills mealybugs is ongoing; we began this work with grape mealybug but were unable to maintain sufficient numbers on vines, and will attempt the work again in 2014 with added improvements.

We conducted several transmission experiments to: 1) compare GLRaV-3 transmission efficiency of mealybugs of different ages; 2) detect GLRaV-3 in various grapevine tissues after first inoculation; and 3) determine latency period between first inoculation and successful transmission of GLRaV-3 by vine mealybug (VMB) crawlers. The results indicate that VMBs of all ages are capable of transmitting GLRaV-3 in grapevines, but that first instar VMBs (crawlers) are the most efficient stage. The detection experiments showed that GLRaV-3 can be detected in potted grapevines 3-4 weeks after inoculation and crawlers can acquire and transmit GLRaV-3 from newly inoculated vines 2 weeks after first inoculation. The findings indicate that the infected grapevines could serve as a virus source long before the appearance of GLD symptoms. The stage-specific transmission studies showed that VMB (crawlers) are the most efficient stage and based on biology of VMB crawlers are also the most dispersible stage. Therefore, the GLD management efforts should focus primarily on effective control of VMB crawlers. However, previous studies have shown that application of even some of the highly effective insecticides after VMB infestation may not prevent GLRAV-3 inoculation by VMB crawlers in newly infested blocks. In this situation, application of fast acting products in a proactive manner would be the best approach to manage GLD in vineyards.

Grapevine leafroll disease, caused by a complex known as grapevine leafroll-associated viruses, is a worldwide threat to vineyard health and sustainability. Mealybugs are the main agents (vectors) responsible for virus movement between vines in the field. This project continues to study the relationship between mealybug populations and the incidence of leafroll disease in vineyards. The ultimate goal is to develop and deploy best management practices for leafroll disease and vineyard mealybugs as vectors of leafroll viruses. The project not only focuses on development of these practices, but also works with grape growers to use these tools in a disease management program. For example, we have successfully developed the use of male GMB traps as a monitoring and decision-making tool and provided significant formal and informal educational efforts on the use of these traps. Traps are more sensitive detection tools that can supplement labor-intensive ground surveys. We provided training sessions on trap deployment and male mealybug identification to ensure that the traps are being used correctly, at the appropriate time of year, and that the trap data are useful to growers. Grape growers are increasingly adopting the use of GMB traps as a management tool. We have also worked with growers to explore and implement other effective and timely management practices, such as insecticide sprays and vine removal.

Our preliminary analysis has shown that prior infection rates of leafroll disease and mealybug populations in the current year will affect the number of vines that develop leafroll disease symptoms in the following year. This has implications for vine removal programs: in a given year, there may be a percentage of vines that contain virus particles but are not showing disease symptoms. This and other complementary projects worldwide are developing best management practices for leafroll disease management that include monitoring and management of vector populations, identification and removal of diseased vines or vineyards, planting of material free of known grapevine pathogens, and regional approaches to management. In future studies we propose to continue to develop new practices, fine-tune current practices to determine when, where and how they can be used most effectively, develop a regional approach to management, and strengthen our educational efforts.

Grapevine leafroll disease is a threat to vineyard health and sustainability worldwide. Mealybugs are the main vector of consequence. This project continues to provide critical information on the relationship between mealybug populations and incidence of leafroll disease in commercial vineyards. In 2011, we completed our 2nd year of data collection in fourteen, 10 acre study areas in Napa County. Near harvest time in each of the previous two years, we have collected data on (1) mealybug populations in grape clusters and (2) symptoms of leafroll disease incidence. Over the course of the study period, this work will accurately map chronological changes in vector density and location with reference to leafroll disease incidence. This study is the first of its kind; ultimately, differences in leafroll disease incidence among sampled vineyards can be used to model leafroll disease epidemiology, as related to vector density and location, virus species, and vineyard cultural practices. A similar study with a collaborator in Oregon is modeled on this work. These data, in combination with information on virus transmission and mealybug control strategies, will facilitate the development of management strategies for grapevine leafroll disease. We are also developing the use of sticky traps as sampling tools. Baited with pheromone lures, these traps attract and capture male grape mealybugs, and are used to follow the flight patterns of the male. Consistent with previous field observations on sessile grape mealybug populations, we have verified that there are two male grape mealybug flights per year in Napa County. Peak flights occur in June and September/October, respectively. Sticky traps have the potential to be more sensitive than field surveys for detecting small populations of grape mealybug. As historical trap catch data are collected, they may also be used to track changes in mealybug populations over time. Additionally, trap catch data may be used to predict events in the life cycle of the grape mealybug. The ability to accurately predict these events is essential for appropriate timing of insecticide applications. In 2012 and 2013, we propose to work with Dr. Daane to develop a predictive model for grape mealybug development, similar to work that was completed for the invasive vine mealybug. Once laboratory studies of grape mealybug development are complete, we will use the trap catch data generated in this study to verify the development model under field conditions. Once developed, the model has the potential to allow growers to predict events in the life cycle of the grape mealybug, eventually leading to optimal timing of insecticide applications on a field-scale.

The 2006-2010 vine mealybug studies were funded, for one or more seasons, by the American Vineyard Foundation, Central Valley Table Grape Pest and Disease District, California Table Grape Commission, California Raisin Marketing Board, Viticulture Consortium West, and CDFA?s Biological Control Program. Sustainable control research focused on development of a mating disruption program and improving biological controls. The potential for mating disruption was initially studied from 2004-2006, with field experiments located in the Central Valley, the Northern Interior Winegrape region, and the Central and Northern Coastal Winegrape regions. Results showed mating disruption, in combination with an in-season insecticide, often reduced numbers of pheromone trap catches (adult males) and crop damage. Development of a commercial program was studied in 2007 and 2008, after which we successfully wrote a Section 18 to allow the commercial sale of mating disruption. In the 2009 and 2010 seasons, we investigated the use of mating disruption with reduced insecticide use. Results suggest mating disruption can be a part of sustainable mealybug control. Keys to its success include pre-conditioning the vineyard to lower mealybug density (e.g., insecticides), multi-year applications of dispensers, early dispenser deployment (e.g., typically May), in-season insecticide applications as needed, and management practices that promote natural enemies. Understanding and improving biological controls of vine mealybug began in 2003, with surveys of resident natural enemies. In 2005, we released commercially available parasitoids from Europe (Leptomastidea abnormis and Leptomastix dactylopii). In 2005, we also began foreign exploration and importation of parasitoids, eventually collected material in the Mediterranean (Spain, Portugal, France, northern Italy, and Sicily), Middle East (Israel, Egypt and Iran), and South Africa. In 2006 and 2007 studies focuses on production and release of A. pseudococci (from northern Italy) and Coccidoxenoides perminutus (from South Africa); ~75,000 A. pseudococci and ~500,000 C. perminutus were released in California vineyards. Recoveries of these parasitoids suggest all four species have established in California vineyards, with A. pseudococci the most important species. In 2009 and 2010, studies investigated A. pseudococci geographic strains, with molecular work showing distinct separation of populations (e.g., eastern Spain compared with Israel). Observations in the insectary and field cage studies suggest that A. pseudococci material from Spain may be best able to suppress vine mealybug populations in California. This parasitoid material has recently been released to commercial insectaries for greater statewide distribution. In 2009 and 2010, we investigated the combined use of mating disruption and biological controls. Studies in commercial vineyards showed little impact of parasitoids, in part, due to low mealybug densities and the use of non-selective insecticide materials. Field studies showed no increase in parasitism on vines with a mating disruption dispenser.