Grape growers have reason to be concerned about the emerging threat of red blotch disease, which was first observed five years ago in California. But, there’s no cause yet to fear a new epidemic, says Deborah Golino, a University of California Cooperative Extension plant pathologist.
The virus associated with red blotch (GRBaV) probably was around long before it was identified last fall using new DNA sequencing technology, she notes. In some cases, it’s even likely that this particular red blotch-associated virus has been the real culprit behind damage to grape quality previously attributed to viruses that cause leaf roll disease.
In fact, some symptoms of red blotch – leaves turning red in early fall and reduced sugar levels in the fruit – are similar to those of leaf roll disease. Unlike leaf roll, though, the leaves with red blotch disease don’t roll and the veins on the underside of the leaves are pink/red in color.
Red blotch virus is the latest in the list of more than 75 pathogens that are spread to grapevines by grafting. However, the virus can show up over time in young, healthy vineyards next to old, infected vineyards. So, it’s possible that a vector may also spread the pathogen, Golino notes.
“Scientists suspect that red blotch disease is widespread wherever grapes are grown,” she says. “The most urgent research need now is to determine how the virus spreads.”
Red blotch disease has been identified among both young (first leaf) and mature (5 to 20-year old) vineyards in California, New York, Virginia, Maryland, Pennsylvania, Texas and Washington. A virus almost genetically identical to red blotch also has been found in Canada.
The red blotch virus, has been detected in Cabernet Franc, Cabernet Sauvignon, Chardonnay, Malbec, Merlot, Mourvèdre, Petite Syrah, Petit Verdot, Pinot Noir, Riesling and Zinfandel. It’s likely that the virus also can be found in raisin and table grapes.
It was first recognized in 2008 in a Napa Valley vineyard by Jim Wolpert, a UC Davis-based Cooperative Extension viticulturist, and Mike Anderson, a viticulture researcher and manager of UC Davis’ Oakville Experiment Station.
However, GRBaV remained unidentified until last fall. That’s when two UC Davis scientists — Mysore Sudarshana , a USDA researcher in the plant pathology department, and Maher Al Rwahnih, a researcher at Foundation Plant Services — discovered it using a new, extremely sensitive laboratory technique known as next generation sequencing.
“We need several years of data to get a firm answer as to how much of a threat the red blotch virus poses to grape growers,” Golino says. “It looks like it can reduce grape sugar levels by as much as 5 degrees brix. Also, it seems that, in some cases, the fruit of infected vines is poorly pigmented.”
Like the human flu virus, some strains of red blotch-associated virus are likely more virulent than others. So, just because the virus is in a vineyard, doesn’t necessarily mean a significant drop in brix readings.”
Like all viral diseases, there is no cure for red blotch, she notes. The only way to prevent the disease is to use virus-free grapevine nursery stock when planting a new vineyard or when replacing infected vines in an existing vineyard.
Golino is director of the Foundation Plant Services at UC Davis. One of its vineyard blocks, the Classic Foundation Vineyard, provides the majority of disease-free scion and rootstock vines to California nurseries. So far, in a testing program scheduled to be completed later this year, only three of 1,600 vines in this vineyard have been found to have the red blotch virus. Another block, the Russell Ranch Foundation vineyard, is used to grow only vines that have been treated to eliminate red blotch and other viruses. Testing of this entire block has found no red blotch virus in any of these vines, Golino reports.
“We’ve made a great start, and we’re well on the way to eliminating GRBaV in nursery stock and being able to provide nurseries a reliable source of root stock that is free of this virus within several years,” she says.
The UC Davis vineyards and those at Washington State University are the two main grape centers of the National Clean Plant Network. Funded by the USDA, this network was created to protect grapes and other specialty crops, like nuts, fruit trees, citrus and berries, from the spread of economically harmful plant pests and diseases.
“We’re very grateful to the USDA for supporting this plant network,” Golino says. “Without its help, the red blotch threat would be much worse than it is.”
Over the next few years the same new DNA sequencing technology used to identify red blotch virus is likely to find other new viruses and microbes, she adds. “Some of those will be disease agents, some beneficial, and some neutral,” she said. “But, the ultimate result will be an increased ability to create superior vineyards.”
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