Courtesy University of California/Agricultural Research ServiceRed blotch virus causes leaf discoloration and lower sugar levels in grapes. In this case, it's cabernet franc.University of California researchers have used DNA sequencing technology to identify the virus that causes red blotch, a disease that discolors grape leaves in the fall and lowers sugar levels in the berries.
Now they need to determine how the virus spreads, according to a news release.
The virus, also known as grapevine red blotch-associated virus or GRBaV, was first found in 2008 in Napa Valley winegrapes.
Although it also affects raisin and table grape varieties, the virus is of most concern to winegrape growers, who have to wait to pick until the berries reach winery specified sugar labels.
In fact, in series cases red blotch virus can reduce Brix levels by up to 5 degrees, according to a National Clean Plant Network fact sheet.
It also has been identified in both young and mature vineyards elsewhere in California as well as in New York, Virginia, Maryland, Pennsylvania, Texas and Washington.
An almost identical virus has been found in Canada.
Scientists can confirm the virus' presence by using PCR, an extremely sensitive genetic test.
"If the vineyard tests positive for red blotch, the grower needs to decide, from an economic standpoint, when is the best time to remove the diseased vines and replant the vineyard," Deborah Golino, director of the UC Davis-based Foundation Plant Services, said in the release. "This is always a complex decision, and there is no one-case-fits-all answer."
The Foundation Plant Services' Classic Foundation vineyard block, from which disease-free scion and roostock vines are propagated, has been partially tested.
It has a very low level of red blotch virus. Only three of the 1,600 vines tested positive.
Test will continue in the coming months.
The new Russell Ranch Foundation Vineyard has been tested with no signs of red blotch virus.
The Naitonal Clean Plant Network has developed a fact sheet on red blotch that can be downhloaded by clicking here.