Red blotch virus, a DNA infection that decreases a grape’s sugar levels on the vine, has been confirmed in Calaveras County, officials said last week.
After confirmed cases of the virus were located recently in El Dorado and Amador county vineyards, University of California Cooperative Extension Farm Adviser Lynn Wunderlich saw what she thought were symptoms of the virus in Calaveras and recommended farmers test for red blotch.
The results came back positive.
Red blotch is a serious threat to wine grape production, although it does not show its signature symptom of changes in leaf color until late summer or fall, about when grapes are being harvested. The virus moves to the base of the plant during dormant periods and moves back to the grape chutes and leaves as the grapes grow.
Visible red blotches appear on vines infected with the virus and there can be a substantial reduction in brix (sugar) levels, which is important as winemakers transform the sugar in grapes into alcohol.
Wunderlich could not confirm how widespread the virus is in Calaveras, where 36 grape varieties are grown on 900 vineyard acres, according to the Calaveras Winegrape Alliance. The virus was only discovered in 2012, but it has spread throughout California and other parts of the United States and Canada since then.
The virus varies symptomatically by variety. In some reds, like Cabernet Franc, sugar development is affected significantly. With white wine grapes such as Chardonnay or Viognier, the impact is much less severe.
It is not clear how many ways the virus can spread, but it has been found to occur in nursery settings where transmission of the virus occurs through alfalfa leafhoppers inside the controlled environment.
“We’re pretty confident the alfalfa leafhopper is one way” the virus spreads, said Wunderlich. “There may be other insect vectors as well.”
Since its discovery, red blotch has been added to certification standards. Growers can check to see whether planting stock they purchase from a nursery has been tested for the virus in conjunction with other standard infections.
According to Wunderlich, no cure for the virus has been discovered or developed. The only recourse available to growers at this point is to remove the infected vines.