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Red Blotch Spread Rate Appears to be Similar to Leafroll 3

Early Draft Research Released by UCD to Address Red Blotch Panic
by Roger Lansing
February 13, 2014

In response to recent concerns and “unnecessary panic“ regarding perceptions on the speed of spread of Grapevine Red Blotch-associated virus (GRBaV) in California vineyards, University of California, Davis (UCD) researchers released a draft research working paper this week to attendees at the annual Current Wine and Wine Grape Research meeting held at UCD February 12 indicating that red blotch spreads no faster than Grapevine leafroll-associated virus 3 (GLRaV-3).

Neil McRoberts, assistant professor in the UCD Plant Pathology Department and the paper’s lead author, said the findings were based on red blotch research at the UC Oakville Experiment Station in Napa Valley, and based on a relatively small number of examples. However, the researchers believe it is important to release these preliminary findings to address concerns about the speed of red blotch spread voiced at the recent Unified Wine & Grape Symposium in Sacramento.

As stated in the paper’s Summary: “Summarizing the results available so far, and using the more familiar, leafroll disease as a reference point, it appears that rates of spread for Red Blotch are similar to those observed for leafroll 3, or perhaps slightly slower, and the level of patchiness observed in vine blocks is also similar to that observed for leafroll 3. Based on the small amount of data available we can say that there does not appear to be strong evidence of very high rates of disease increase, but it is clear that rates will vary from block to block depending on local conditions. Work is in progress to characterize both the rate of spread and the variation in Red Blotch patterns in nursery and production vineyards so that we can provide a more comprehensive guide to detecting and managing the disease.”

The draft working paper, “Predicting the rate of spread and sampling accuracy for Red Blotch disease in grapevine blocks,” produced by the Quantitative Biology and Epidemiology (QBE) Lab in the UCD Plant Pathology Department, will soon be accessible through the Lab’s website at The paper is coauthored by Mc Roberts, UCD plant pathology professor Robert Gilbertson, graduate student and QBE lab researcher A.J. Campbell, UCD viticulture researcher Mike Anderson, and Foundation Plant Services (FPS) director Deborah Golino.

McRoberts also used the opportunity to suggest the industry work together more calmly and efficiently to address the issue, and he provided advice to industry parties regarding what he characterized as “an unpredecented amount of hysteria about the disease.”

To winemakers: “Everybody needs to get their panties out of a bunch. This is something with a new name, it’s not a new disease. You’ve been making wine with grapes that have had this for years. Let’s all be reasonable.”

To growers and vineyard managers: “This is an uncertain time, but try not to pass more anxiety to nurseries and others. We’re all trying to come up with good sampling plans and methods.” He also advised growers not to put pressure on nurseries to propagate uncertified grape vine material. McRoberts noted that all the vine material planted in the new FPS Russell Ranch Foundation Vineyard has been tested and is entirely red blotch virus-free, in addition to following the strict “Protocol 2010” standards of the National Clean Plant Network for virus-free certified nursery stock. “That’s the answer,” he said, and urged patience until this clean and certified plant material can become available in sufficient quantities to work its way into nurseries and vineyards.

To nurseries and growers: “Get your field people trained to identify disease symptoms and start collecting data on where you find it. Use available PCR diagnoses from analytical labs in a sensible way.”

To research scientists: “Don’t get into a turf war about who will work on the problem.”

Insect Vectors Suspected but None Confirmed

Although no specific vectors of red blotch have been confirmed to date, there is belief that insect vectors could be involved. In a presentation at the February 12 research meeting, UCD-based US Department of Agriculture Agricultural Research Service research plant pathologist Mysore Sudarshana noted that researchers in Washington state have shown that red blotch can be vectored by the Virginia Creeper Leafhopper in greenhouse lab conditions, but there is no confirmation this insect has vectored the virus in commercial vineyards in Washington or in California. “In the vineyards I’ve visited where I’ve seen red blotch, I don’t see the Virginia Creeper Leafhopper,” Sudarshana said, who has been studying red blotch since 2008. He said this insect has been found in vineyards in California, but not in locations where red blotch has been identified.

Red Blotch Effects on Fruit/Juice Quality

Sonoma County viticulture advisor Rhonda Smith provided results from an American Vineyard Foundation funded project during 2013 on the effects of red blotch on canopy symptom development and fruit maturity in Cabernet Sauvignon, Merlot and Chardonnay in a small sampling of symptomatic red blotch positive vines in North Coast vineyards. While there was some variation between varieties, Smith said the general results regarding juice chemistry showed that fruit from GRBaV vines consistently was lower in total soluble solids (Brix) ranging from 1 to 3 degrees, higher in malic acid, and higher in titratable acidity (TA). The study also looked at the effects of dropping crop during the season to improve ripening and fruit quality. Smith said, “Reducing crop load in infected vines did not consistently improve juice chemistry.”

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