Another pertinent and somewhat ominous talk that our Technical Manager Suzanne McLoughlin listened to as part of the 2021 CDFA Pierce’s Disease Research Symposium in December, was a case study from Dr Kristin Lowe from Vine Balance Consulting on her experiences in the Napa Valley with Grapevine Red Blotch virus. This virus while currently exotic to Australia, is categorised as a High Priority Plant Pest for Australian viticulture industries.

Kris focused on the struggles that growers are currently experiencing with rogueing for this virus. Suzanne presents key highlights from the presentation below.

Monitoring Red Blotch infection levels over time in one block, Kris reported 0.3% incidence in 2019, 2.6% incidence in 2020 and 26.15% incidence in 2021. In another block, there was 0.04% incidence in 2019, 0.82% incidence in 2020 and 15.77% incidence in 2021. With additional examples, she noted that 2021 was a particularly bad year for Red Blotch levels. With these high infection rates, Kris is also concerned about spread of Red Blotch to adjacent blocks.

Another vineyard she consults to had a 55% infection rate in 2021, with symptoms not evident until the second season after planting.

Based on this experience, Kris is querying:

  • Whether the rapidly increasing infection rate is resulting from natural or vectored spread within the block (knowing that she has seen presence of a vector), or is it more likely to be a result of a latent infection coming from the planting material
  • Why the infection rate is so fast
  • (How) can such changes in infection levels be effectively rogued
  • How many of the ‘apparently healthy’ vines are really healthy
  • What testing strategy can be used to gain confidence in the true level of infection and when should the testing be undertaken

“One factor Kris has become aware of in the Red Blotch virus fight is that the virus spread is ahead of visual symptoms. Asymptomatic vines are testing negative and are then naturally retained in the block, but then are testing positive the year after, acting as a source of spread for the virus,” Suzanne said.

“In another example, rogueing was started in 2018 but ultimately annual rogueing practices couldn’t keep up. Red blotch infection was at an incidence of 3% in 2018 in this block, but by 2020 was at 11% and then at 20% in 2021; by which time after harvest, a decision was made to grub the block.”

Kris stressed that rogueing for grapevine Red Blotch virus is hard work and worth it in some situations but not in others. Her tips for managing Red Blotch virus infections where rogueing was practiced were:

  • You must source clean replants, sometimes in small numbers (which nurseries may not like).
  • You must walk the vineyard block multiple times per season to identify and account for variable disease expression in individual vines.
  • At some point, you must accept that you won’t identify (and therefore be able to rogue) all diseased vines.
  • You must mark suspect vines (using flagging tape, spray paint, or other means).
  • You must remove vines individually, waiting for optimal soil conditions and accounting for disposal issues.
  • You must take care of replants – accounting for variable water, nutrient and training needs.

Kris stated that there are many facets of the Red Blotch virus issue that keep her up at night:

  • Assuming that removal of infected vines is integral to controlling the rate of spread of the virus, how should the removal be done, when and which vines should be removed?
  • Should a grower wait to replant missing vines until infection levels start to drop in a block, but what if they don’t?
  • Should asymptomatic vines be tested to determine a better vine removal pattern?
  • Is there any published data showing that rogueing works for Red Blotch virus?
  • What chance to do we have if at least one vector (three-cornered alfalfa hopper) can hold onto the virus for 30 plus days, across nymph stages, they fly, and symptoms don’t show up for months or years later?
  • Area-wide management must play a role.
  • How clean is our planting material and how do we keep it clean?

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 Red Blotch virus 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 Grapevine Red Blotch virus are underway and are likely to provide outcomes relevant to the Australian situation. Some of the projects are outlined below.

Monitoring Grapevine Red Blotch Virus at Russell Ranch Foundation Vineyard

Project Leaders and Cooperators: Maher Al Rwahnih, Neil McRoberts, Deborah Golino, and Vicki Klaassen, University of California, Davis; Kent Daane, University of California, Berkeley; and Houston Wilson, University of California, Riverside

Establishing new vineyards with virus-tested, certified planting material will continue to be an essential component of Grapevine Red Blotch Virus (GRBV) control. However, the introduction of GRBV at Russell Ranch from outside sources demonstrates that starting with clean planting material may not be enough to stop GRBV from entering vineyards at some point. Once GRBV has been introduced into vineyards, our work indicates that spread can be rapid with annual rates up to 18%.

GRBV distribution within relatively newly infected vines is highly uneven, making it difficult to detect. These vines almost certainly contribute to false negative test results in any given year’s testing and could serve as inoculum sources for transmission that same year if a vector is present. Determining how quickly these infections can be reliably detected requires field transmission experiments with a known vector.

We hope to have a more accurate estimate of annual spread rates in areas of highly aggregated GRBV infections as we continue to test the sentinel vines. To date, we have not detected GRBV in these vines. However, during sampling in October and November 2021 we did notice girdled petioles, which indicates that three-cornered alfalfa hopper is present and feeding on these vines.

While it’s clear that GRBV has continued to spread at Russell Ranch, we did not gain any specific insights on a vector from this work. Spatial-temporal analyses for 2017-2020 indicates that spread is occurring via a vector that is more mobile than mealybugs.

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. Prioritizing 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 organizations 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

Developing a GMO-Free RNA Interference Approach to Mitigate Red Blotch Negative Impacts on Grape Berry Ripening

Project Leader and Cooperators: Laurent Deluc and Jeffrey Nason, Oregon State University; Keith Lloyd Perry, Cornell University; Robert Martin, United States Department of Agriculture; and Denise Dewey, Oregon Wine Research Institute

RNA interference (RNAi) is a conserved biological response across living organisms (animal or plant cells) initiated by the presence of double-stranded RNA molecules from various pathogens, including viruses. The RNAi mechanism initiated in the plants will lead to a cascade of molecular events that are meant to repress the activity of the virus and its propagation within the plant. Once infected, the plants will recognize and produce specific nucleic regions of the viral genome to activate the RNA silencing machinery. These regions are named ‘hot spots’.

The main goal of this project is to identify these ‘hot spots’ of the grapevine Red Blotch virus. In the long-term, this knowledge could help develop innovative technology tools like ectopic RNA molecule application in vineyards to mimic the virus’s presence and make the plants immune or primed to further infections like a vaccine will do. The team expects to have all the data necessary by the end of winter and to have the ‘hot spots’ identified by the end of spring 2022.

Ecology of Grapevine Red Blotch Virus

Project Leaders and Cooperators: Marc Fuchs and Keith Perry, Cornell University and Deborah Golino, University of California, Foundation Plant Services

Grapevine Red Blotch Virus (GRBV), the causal agent of Grapevine Red Blotch Disease, is transmitted by grafting and by the threecornered alfalfa hopper, although this insect is not a pest of grape. Our research shows the threecornered alfalfa hopper’s active role in transmitting GRBV in the vineyard. We documented 10 days of insect exposure to infected grapevines are necessary for the virus to be transmissible to healthy grapevines. We then revealed transmission of Red Blotch virus from and to free-living grapevines by the threecornered alfalfa hopper. Next, the ability of the threecornered alfalfa hopper at transmitting GRBV in the vineyard was shown as early as three-months post-exposure of insects carrying the virus to healthy grapevines in an experimental vineyard. Test vines that became infected via vector-mediated inoculation are continuously monitored, as they have not exhibited disease symptoms yet.

Additionally, an accurate, cheap, and user-friendly GRBV diagnostic assay was developed and validated with winegrape growers in Napa and Sonoma counties. Some of these growers have adopted the assay for on-site diagnosis to determine GRBV incidence in vineyards. Research progress and information on disease ecology was communicated to grower communities.

Investigating the Impact of Grapevine Red Blotch Virus on Grape Skin Cell Wall Metabolism and Soluble Pathogenesis-Related Proteins in Relation to Phenolic Extractability

Project Leader and Cooperators: Anita Oberholster, Mysore Sudarshana, Larry Lerno, and Cristina Medina Plaza, University of California, Davis

Our research indicates that Grapevine Red Blotch Virus (GRBV) causes a delay in ripening events in grapes, leading to significant decreases in sugar accumulation, color development, and aroma compound accumulation. In addition, our work has indicated that GRBV affects key metabolic pathways that are responsible for the production of compounds (phenolics) that are important to the color, flavor, and mouthfeel of a final wine. However, there is little known about the impact of GRBV on cell wall composition and structure. It is known that during ripening the grape cell wall changes in composition and integrity, which impacts phenolic extractability during winemaking. In addition, it has been shown that pathogens such as fungi, bacteria, and viruses alter cell wall modifications. Consequently, these changes in the grape cell wall can directly impact the extractability and final concentrations of phenolics in wines.

Our current research indicates that grapes infected with GRBV have significantly higher quantities of pectin (acidic heteropolysaccharide groups located in the cell wall) and soluble proteins. Previous studies have shown that higher levels of pectin and soluble pathogenesis related (PR) proteins result in binding reactions to phenolic compounds such as tannin. Therefore, studying the impact of GRBV on PR proteins, cell wall composition, and cell wall enzymatic processes will aid in understanding plant-virus interactions and potential mitigation strategies to alleviate the impact of GRBV on grape composition and wine quality.

Effects of Grapevine Red Blotch Disease on Flavor and Flavor Precursor Formation in the Grape and on Wine Quality

Project Leaders and Cooperators: Michael Qian, Alexander D. Levin, James Osborne, Elizabeth Tomasino, and Achala KC, Oregon State University; Michael Moore, Quail Run Vineyards; and Randy Gold, Proprietor, Pacific Crest Vineyard Services

Two irrigation treatment main plots were randomized in two blocks of fields and characterized by varying water application rates (wet and dry) on both red blotch infected (RB+) and non-infected (RB-) Pinot noir grapevines. A wet treatment was irrigated at 100% estimated crop evapotranspiration (ETc) and dry treatment was irrigated at 66% ETc. The impact of grapevine red blotch disease on grape and wine quality was studied across three years. Pinot noir grapes were collected during berry ripening until the harvest (one week after harvest in 2019 and 2020) from RB+ and RB- grapevines. Wines were made from Pinot noir grapes with treatments included D+ (dry treatment on RB+ grapevines), D- (dry treatment on RB- grapevines), W+ (wet treatment on RB+ grapevines), and W- (wet treatment on RB- grapevines) conducted through 2018 to 2020.

Berry maturity parameters, berry free and bound form C13-norisoprenoids, wine anthocyanins, phenolics, and flavor profiles were investigated. The results indicated that infected grapes had a lower level of total soluble solids at harvest. Wines made from infected grapes showed lower total phenolic content compared to wine from noninfected grapes. Certain volatile compounds can be affected by both the health status and irrigation treatments of grapevine. Wet treatment may enhance the levels of some volatile compounds in RB+ wines based on the result shown in 2019 and 2020. The patterns were not consistent throughout the years, which suggested that vintage was also an important factor in the volatile wine profile.

Structure-Function Studies on Grapevine Red Blotch Virus to Elucidate Disease Etiology

Project Leaders and Cooperators: Christopher D. Rock and Sunitha Sukumaran, Texas Tech University; David Tricoli, University of California, Davis; Rhonda Smith, University of California Cooperative Extension; and Achala KC, Oregon State University

This project aims to establish proof of concept that genetic modification of genes involved in Grapevine Red Blotch Virus (GRBV) expression may change the disease etiology. The research targets diffusible signals (viral suppressor proteins) and host target small RNAs directed to GRBV C2 and V2 genes, and aims to develop chemical inducers of host resistance. It is likely that microRNA and trans-acting small interfering RNAs generated from hairpin viral suppressor targets operate systemically by moving through vasculature. If proven, this research has applications to develop genetically engineered grapevine rootstocks for GRBV resistance to use with non-genetically modified organism grafted scions.

Improved Understanding of Virus Transmission of Grapevine Red Blotch Virus

Project Leader: Vaughn Walton, Professor, Oregon State University

It was previously believed that treehoppers are the key insect species transmitting Grapevine Red Blotch Virus (GRBV). Our work was conducted with extreme rigor, ensuring that there was no cross contamination of insect tissue or feces on plant tissue, possible sources of contamination and false positive tests. Our data illustrate that it is possible for the threecornered alfalfa hopper to vector GRBV, but that this transmission happens in rare cases. None of the other tested treehopper species were able to vector GRBV to grapevines. These data therefore point to the fact that there must be other arthropod species possibly vectoring GRBV.

Biology and Role of Treehoppers in Grapevine Red Blotch Disease

Project Leaders and Cooperators: Frank G. Zalom, Mysore R. Sudarshana, Kaan Kurtural, Cristian Olaya, Teresa Erickson and Michael Bollinger, University of California, Davis; Kent Daane, University of California, Berkeley; Rhonda Smith, Cindy Kron, Lynn Wunderlich, and Cindy Preto (Kron), University of California Cooperative Extension; and Vaughn Walton, Oregon State University

The results of this project added significant new knowledge towards better understanding the role of the three-cornered alfalfa hopper and other vineyard treehoppers in the epidemiology of Grapevine Red Blotch Virus (GRBV), including management of virus spread, by determining feeding on grapevines seasonally and their phenology in relation to covercrops and non-crop vegetation in and around vineyards. Possible transmission by other treehoppers, planthoppers, and phloem-feeding leafhoppers found in vineyards where GRBV is spreading has also been studied. This essential information will contribute to the management of grapevine Red Blotch disease by cultural methods such as reducing plant hosts favorable to sustaining vector populations or precise treatment timings based on treehopper biology in vineyards, and when transmission is most likely to occur.