The California Department of Food and Agriculture (CDFA) hosted a Pierce’s Disease Research Symposium in December 2021, and our Technical Manager Suzanne McLoughlin attended online.
One of the most pertinent presentations was from Aaron Lange, Vice President of Vineyard Operations at LangeTwins Family Winery & Vineyards, who delivered a case study on his experiences in managing Grapevine leafroll-associated virus 3 (GLRaV-3) on his own vineyards and clients’ vineyards in the Lodi Region of California.
“Aaron described managing GLRaV-3 in the presence of mealybug as a ‘daunting issue’ and ‘life-changing’, and because of his personal experiences, has recently established a neighbourhood cooperative vector and virus control program for GLRaV-3,” Suzanne said.
“The focus of Aaron’s management of GLRaV-3 has centered around controlling the mealybug vector to limit the spread of the virus, and to ensure planting with virus-free material. He presented results from several blocks to demonstrate the huge scale of the situation he and others are facing.”
Suzanne presents key highlights from the presentation below.
Grapevine leafroll virus infected block examples
One block planted on virgin ground (Cabernet Sauvignon Clone 8/Freedom rootstock) in 2013 started showing red leaf symptoms in 2015. Subsequent laboratory testing indicated the presence of GLRaV-3. They didn’t know where the virus had come from, so began an intensive program of visually scouting for symptomatic vines of GLRaV-3 followed by laboratory testing of a portion of suspect positives to verify the visual detections. Each year, 10 suspect random vines were laboratory tested to correlate to the visual assessments and no false positives were ever detected – thus a good correlation between visual scouting and laboratory results.
All positives were mapped to enable analysis of spread over time. The following year in 2016, 8.1% of the block was identified as infested with GLRaV-3 and the spatial distribution of these infected vines was considered to indicate likely infected planting material from the nursery. In 2017, scouting, testing, mapping and rogueing of infected vines was undertaken.
The rogueing program involved removal of all visually positive GLRaV-3 vines in the block at the time. In both 2017 and 2018, the incidence of rogued GLRaV-3 vines was 1.2%, in 2019 it was 1.5% and in 2020 it was approximately 1%. They are still unsure if the virus spread in this block was compounded from mealybugs being blown in from an adjacent vineyard block to the north.
Aaron also reported the incidence of year-on-year rogueing in another client’s block with the same planting material (Cabernet Sauvignon Clone 8/Freedom rootstock) also planted on virgin ground in 2013 – of 3.3% in 2017, 4.4% in 2018 and 4.1% in 2019. At this level of rogueing, Aaron commented that it was hard to demonstrate economic viability of this practice – both in the time and cost spent in rogueing and subsequent reduction in productive vines left, and the relatively low grape prices received in the area.
In this block, they also noted higher density of mealybugs in vines adjacent to trees and suspected that bird spread of mealybug was occurring. By 2020, the incidence of vines requiring rogueing reduced to 1.1% but then increased sharply to 4.7% in 2021 (equating to 1,877 vines).
Aaron said this latest increase was a depressing result as they thought with all the effort they had put in over five years, they had the GLRaV-3 virus under control. After the elevated 2021 result, the client decided to discontinue the rogueing program due to the poor economics, and to attempt to control the virus and vector in other ways. In this block, they suspect the initial virus was present in the planting material from the nursery and that mealybugs also blew in from neighbouring vineyards, exacerbating the spread.
A collaborative approach to addressing GLRaV-3 and mealybug
Aaron recognised the importance and potential benefits that could be realised through a collaborative neighbourhood effort in addressing the GLRaV-3 and mealybug issues. He recently led an alliance of 21 vineyard operators covering more than 1,300 hectares of vineyard in an area-wide program, gaining agreement that:
- A neighbourhood alliance was needed
- Leafroll 3 was not to be ignored
- Vine mealybug was spreading this virus rapidly
- A neighbour’s mealybug problem is your problem: it often blows into your vineyard
- While a little bit of GLRaV-3 is OK, in the presence of a ubiquitous vector, the potential for spread is enormous
This alliance identified several barriers to collaboration which would be commonplace everywhere, including:
- Differing levels of knowledge and expertise
- Differing thoughts about rootstocks and their resistance to pests and diseases
- Differing economic/financial situations
- The stigma of individuals admitting they had a problem
- Feelings of futility and helplessness
- Not wanting to feel judged
The group has begun to take a series of actions including:
- Installation of mealybug traps to attract males (e.g., Suterra’s Small Paper Delta Trap with VMB Septa Lure):
- Trap counts are recorded every two weeks and results spatially mapped and shared openly within the group. These have provided elevated awareness of the issue and responsibility from some to the regional spread issue.
- Application of pheromone-based mating disruptors (e.g., Suterra’s CheckMate® VMB-F sprayable pheromone), which to date have proven very effective, with those properties using the pheromone disruptors not showing any males caught in the traps.
- Chemical regimes from all properties are being monitored and in time will be included as part of the program.
“While this area-wide approach is still in its infancy, the level of virus infection experienced certainly points to this approach being an extremely valid one,” Suzanne said.
“Aaron identified there is still considerable work to be done in understanding this issue, and while this is certainly the case, we are lucky in Australia to have the opportunity to keep abreast of learnings from the US as similar virus and vector issues become more prominent here.”
Questions Aaron says remained unanswered, which also relate to our situation here in Australia, include:
- What is the source of the GLRaV-3?
- Neighbouring vineyards vectored by vine mealybug?
- Nursery stock?
- Is the GLRaV-3 predominantly coming from the scion or the rootstock? If the scion, is it all detectable by PCR testing? If the rootstock, is it all detectable by PCR or could the rootstock be ‘hiding’ the virus.
- What about the soil – are virifulous vine mealybugs staying in the soil potentially on remaining vine roots after vines have been removed and then reinfecting new vineyards?
- How can we tell the difference between latent virus expression versus a new vectored infection?
- Are we doomed to forever manage both the vector and the virus, and can we survive it?
- How effective are pheromone traps to attract male mealybugs?
- Will there be more options for insecticide control of mealybugs?
- What support is there for cheap, large area-wide mating disruptor control?
- Exponential improvement in the economics of detecting and testing for GLRaV-3 is needed.
This information was provided as part of the annual CDFA Pierce’s Disease Research Symposium held online in December 2021. We sincerely thank the CDFA for making this symposium available worldwide.
New Grapevine leafroll virus and mealybugs research projects
The Pierce’s Disease and Glassy-Winged Sharpshooter (PD/GWSS) Board in the US invests in research and outreach to prevent the spread of pests and diseases and deliver practical and sustainable solutions. According to the CDFA and California PD/GWSS Board’s Pierce’s Disease Research Projects at a Glance December 2021 report, a number of research projects relating to Grapevine leafroll virus and mealybugs are underway and are likely to provide outcomes relevant to the Australian situation. Some of the projects are outlined below.
Genomics Resources for Identification, Tracking, Surveillance, and Pest Management of Vine Mealybug in Vineyards
Project Leaders and Cooperators: Lindsey Burbank, Rachel Naegele, and Mark Sisterson, United States Department of Agriculture, Agricultural Research Service; Dario Cantu, University of California, Davis; and Kent Daane, University of California, Berkeley
Continued use of chemical control for insect pests such as mealybugs is likely to lead to development of insecticide resistance. It is necessary to explore alternative control strategies based on a detailed understanding of pest biology. DNA sequence information for vine mealybug will enable development of new pest control technologies. This project will expand DNA sequence information for vine mealybug representative of pest populations across California. This information will be used to track pest populations, evaluate prevalence of insecticide resistance, and develop new pest control technologies based on novel genetic targets. A high-quality vine mealybug reference genome was created from single insect DNA extraction. Work is ongoing to annotate the reference genome using RNA sequence data from male and female vine mealybugs. Although little genetic diversity was observed initially based on microsatellite markers, new genomic references will be used to screen for additional marker candidates.
Improving Extension Outcomes: Identifying Drivers and Barriers to Adoption of Management Practices Using Leafroll and Red Blotch Disease as Model Systems
Project Leaders and Cooperators: Monica L. Cooper, Malcolm B. Hobbs, and Larry Bettiga, University of California Cooperative Extension; Stephanie Bolton, Lodi Winegrape Commission; and Michelle A. Moyer, Washington State University
Grapevine Leafroll Disease and Grapevine Red Blotch Disease are consequential viral diseases of grapevine that are actively managed by wine grape industry professionals in the western United States. Uptake and implementation of management practices varies among growers and across regions. To understand why, we conducted a survey and interviews with growers in California and Washington and collected feedback from decision-makers on educational resources.
Economic, technical, and social-behavioral factors influence the adoption of management practices for grapevine leafroll and red blotch diseases. The cost of practices – in time, labor, and outlay – are important considerations, as are production demands and salability of product. Specifically, yield, quality and grape pricing were contributing factors. Therefore, individual or collective practices that reduce the economic burden of adoption can improve regional disease management outcomes. The most influential technical factor is the availability and acceptance of evidence-based management practices. Prioritising research and outreach programs that develop and disseminate an evidence-based understanding of disease ecology and management can reduce the detrimental effects of viral diseases. Lastly, programs that improve regional camaraderie and collaboration, as well as supportive learning environments within individual organisations are social factors that can increase adoption of management practices.
Improved Decision-Making for Grapevine Leafroll and Red Blotch Diseases Using Rapid Identification Tools and a Regional Approach to Monitoring and Management
Project Leaders and Cooperators: Monica L. Cooper and Jennifer K. Rohrs, University of California Cooperative Extension; Tom Shapland, Tule Technologies; Rodrigo P.P. Almeida, University of California, Berkeley; Kar Mun Chooi, The New Zealand Institute for Plant and Food Research; and Keith L. Perry, Cornell University
Since Grapevine Leafroll Disease (GLD) and Grapevine Red Blotch Disease (GRBD) are incurable, mitigation efforts to reduce spread include (a) sourcing virus-screened plant material; (b) removing diseased vines individually (roguing) or redeveloping high incidence blocks; and (c) reducing vector populations. Successful roguing requires accurate identification of diseased vines, which can be challenging when symptoms are confusing, asynchronous, or absent (such as in white-berried cultivars). This project seeks to increase the accuracy of visual assessments and improve vine removal efforts using artificial intelligence (AI) and an ‘in-house’ assay. We will also address fundamental questions of GRBD ecology by harnessing the power of grower-collected data. Regional monitoring of the disease and vector, combined with network-based learning, will address uncertainties in GRBD epidemiology and management. Educational opportunities will include field days, workshops, seminars, and networking groups. Educational products will include handouts and instructional videos. This project will advance the use of emerging technologies to identify diseased vines and support grower-coordinated efforts to reduce the economic and environmental impacts of GLD and GRBD for the grape industry.
Visit the project website at https://bit.ly/ucce-red-blotch.
Resistance to Grapevine Leafroll Virus 3 and its Major Mealybug Vectors
Project Leaders and Cooperators: Marc Fuchs and Greg Loeb, Cornell University and Deborah Golino, University of California, Foundation Plant Services
The objective of this research is to develop grapevines resistant to Grapevine LeafrollAssociated Virus 3 (GLRaV-3), the dominant leafroll virus in diseased vineyards, and to the grape mealybug and the vine mealybug, the two most important vectors of GLRaV-3 in California vineyards, using RNA interference (RNAi).
Preliminary results showed that mortality of grape mealybug nymphs significantly increased after feeding on an artificial diet supplemented with double strand RNA (dsRNA) constructs against mealybug genes. Predictive modeling suggested that one of the dsRNA constructs against the grape mealybug is unlikely to provide any cross-reactivity against the vine mealybug. Therefore, we engineered a new dsRNA construct against the counterpart gut gene of the vine mealybug. This new RNAi construct was identified and characterized from dissected gut tissue of vine mealybugs established on winter squash in a growth chamber. The integrity of this new dsRNA construct was validated. It will be used in combination with the three dsRNA constructs from the grape mealybug for optimal RNAi efficacy.
For resistance to GLRaV-3, target dsRNA constructs were developed and used in grape transformation experiments for the recovery of stable transformants. Some transgenic rootstock 110R and 101-14 and Vitis vinifera cultivars Cabernet franc and Pinot noir were obtained and characterized. Transformation efforts are pursued by using stacked dsRNA constructs against the mealybugs and GLRaV-3, and by directing their expression in the phloem tissue using a tissue specific plant promoter. The production of grape plants stably transformed with dsRNA constructs against GLRaV-3 and the two mealybug pests is advancing.