Among the dozens of different viruses that like to attack vines, the acronym GRBV ranks at the top of a list of some 85 viruses known to infect those vines — more than any other perennial crop.

A recent report by the National Clean Plant Network identified grape red blotch virus as one whose origin is unknown but was first noticed on Cabernet Sauvignon in Napa Valley (the UC Davis Oakville Research Station) some 15 years ago.  As NCPN notes: “There’s a good correlation between the presence of GRBV and red blotch symptoms, but correlation does not prove causality.”

Much like leafroll disease, vines with red botch show similar symptoms of leaves turning red at the base of the shoots with pink/red veins on the undersides of leaves.

The host range is becoming extensive in both red and white winegrapes as well as table and raisin grapes and rootstock. Current distribution is extensive and in California includes vineyards (both first leaf and mature) in the North and Central Coasts as well as San Joaquin Valley.

The virus spreads through grafting and propagation of infected plant material and by insect vectors according to the California Department of Food and Agriculture that puts blame on the three-cornered alfalfa hopper. Oregon State University researchers report viral transmission in laboratory and greenhouse studies “with other treehopper and leafhopper species found in and around GRBV-infected vineyards which may contribute to the virus spread.”

Vector transmission trials at University of California, Davis have shown GRBV transmission by the three-cornered alfalfa hopper to be 20% in test plants.

Like other viruses, reports NCPN, “Once it is present in a vineyard, there is no cure,” and CDFA reports potential damage as “a reduction of sugar accumulation, an increase in malic acid, and less-consistent increases of pH and titratable acidity. Cluster weight may be reduced as may the yield varying by cultivar.”

Loss of sugar

USDA and Oregon Department of Agriculture experts estimate a significant loss of sugar accumulation is possible, with “up to 5 Brix reported depending on variety, rootstock, and location.”

The topic was serious enough for UC Davis to sponsor a Grapevine Red Blotch Disease Symposium recently with experts from that school and colleagues from UC Riverside and UC Berkeley presenting the latest scientific findings.

“We’ve learned that if the whole vineyard is infected, it was probably planted there in original stock, while if only some vines are infected, a vector is suspected,” said Frank Zalom, UC Davis distinguished professor of entomology.

“Our transmission studies looked at potential patterns of spread, like tree hoppers and leaf hoppers with the three-cornered alfalfa hopper the most likely.”

Zalom said reports of the virus have come from growers as far south as the Temecula area, adding, “While there hasn’t been a systematic-type survey across the state, basically it’s all over, probably achieving the large distribution because it originally was spread by infected nursery stock.”

Asked about prospects for further red blotch spread this season, he said: “If a vine is infected, it never loses the virus, so any spread would be a function of the vectors. To get rid of it, you need to remove the vine because no cure exists.

“There is some cultural management possible by reducing vector populations on the ground cover, be it cover crop or weeds. They’re in there right now, feeding on ground cover and depositing immature carriers that will move up the vine as they approach adulthood sometime in mid-to-late-May. At that point, vector adults transmit the virus to uninfected vines.

“Try to reduce that population early in the season by eliminating the overwintering host for the immatures — before they become adults.”