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Researchers Confirm Red Blotch Vector

April 2016
by Andrew Adams and Jim Gordon
Brian Bahder (left) and Frank Zalom with the University of California, Davis, look for insects at a red blotch-infected vineyard in Amador County.

San Rafael, Calif.—A team of researchers with the University of California, Davis, identified the three-cornered alfalfa treehopper (Spissistilus festinus) as the first confirmed vector of red blotch-associated virus in greenhouse tests.

The researchers cautioned that the treehopper might not be the only vector. “It’s important to note we’re not specifically saying this is the only vector out there,” said UC Davis entomology post-doctorate researcher Brian Bahder during a Feb. 26 webinar presentation about his work. The webinar, which features several other updates about current red blotch research, is available online.

Bahder has been studying California vineyards with evidence of the virus spreading through them starting with the UC Davis Oakville research vineyard. Researchers, however, also have found vineyards in California’s Amador and Santa Barbara counties that exhibit the telltale signs of how it’s spreading, such as a cluster of infection on the edge of a vineyard that spreads to other vines.

Bahder has been surveying the insect populations of these vineyards to see which critters are present in all of them. “I’ve been able to come up with a short list of species that I believe are likely going to be the vector of this virus,” he said.

That short list is comprised of one treehopper (Membracidae), two leafhoppers (Cicadellidae), one jumping plant lice (Psyllidae) and one planthopper (Cixiidae). Bahder took one insect from each of these species and placed it in a greenhouse with infected vines for 48 hours, and then placed it with clean plants for another 48 hours. During the webinar he discussed the results after five months since the trials.

To test for the presence of the virus, Bahder used digital PCR analysis that he said is far more sensitive than quantitative PCR, and which can detect the virus at levels of much lower concentration.

Of Bahder’s short list of suspects, he only was able to reproduce transmission of the virus with the three-cornered alfalfa treehopper. Grapevines are not the insect’s preferred host, which are grasses, legumes and its namesake alfalfa. Bahder said such plants could often be on the edges of vineyards in riparian areas, so that could explain the pattern of the spread of the disease in some vineyards.

Bahder and the other members of the research team are working on a paper about the findings and continuing the research. “We have to follow up on the species and look out for the true ability of this insect to vector in a field setting as opposed to greenhouse conditions,” he said.

The discovery of a vector garnered the attention of the wine industry from a blog post by Lynn Wunderlich, the UC Cooperative Extension advisor for the Sierra Nevada Foothills region.

Wunderlich is part of a team of advisors and researchers that has been monitoring and mapping vineyards with a pattern of red blotch spreading. Other team members include Rhonda Smith in Sonoma County, Mike Anderson at the Oakville research station in Napa County and virologist Mysore Sudarshana.

She said she’s seen evidence of the three-cornered alfalfa treehopper in the Foothills for years from the damage it can do to vine leaves. “Looking for the telltale ‘single red leaf’—always with the petiole girdled—is one fairly easy way for growers to see if the treehopper is present in their blocks,” she told Wines & Vines. “That said, there is much we don’t yet understand about this insect and so it’s really too early to give any sort of recommendation.”

She said pulling infected vines is still the best option to prevent the spread of the disease. “Roguing infected vines is critical to reducing spread,” she said. “Most of the red blotch I’ve seen in the Foothills has a pattern that suggests it came in with planting material.”

The disease and associated virus were also the subject of one of the most well attended sessions of Wine Business Monthly’s Innovation + Quality conference March 3.

Moderator Celia Welch, a winemaking consultant in Napa Valley, said she recently was dealing with a vineyard block that just never reached its potential. The vines were not showing symptoms; the grapes reached a good Brix level, but other quality properties were lacking. Welch had the block tested and it was 90% red blotch.

“I really did not want to have to face another vineyard crisis in my career,” Welch said. “After phylloxera, after fan leaf virus, after European grapevine moth, I just thought, ‘Damn it, why me? Why here, why now?’ But it’s around, there’s no shame in it. Let’s get into this, let’s get it behind us.”

Jim Barbour, a Napa Valley vineyard management consultant, described his experience with a Rutherford, Calif., vineyard that was planted in 2012. Barbour said he adopted a zero-tolerance policy to “rogue out” individual infected vines right after harvest. He added that his nursery supplied clean material free of charge for replanting. The zero-tolerance approach seemed to be working, and Barbour cautioned the audience to remember that negative test results can be wrong on the full virus panel.

Alan Wei, owner and general manager of Agri-Analysis in Knights Landing, Calif., reminded the audience that the leafroll 3 virus is still the most damaging.

Wei said that in his company’s testing, more infected vines turn out to have red blotch rather than leafroll, and that red blotch has been found in wild, free-growing vines. He said the virus is graft transmissible, and it appears the virus can also spread from the scion down to the rootstock. He said he hopes further research will determine if the virus is spread by other vectors and what those are.

Wei encouraged growers to question their grapevine suppliers in detail about their certified stock, “because certified stock is supposed to be clean, but as many of you know, it is not always.”

He recommended that nurseries use a higher rate of sampling and a greater frequency of sampling to keep planting materials free of the virus.

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