Over the last year, red blotch virus has been detected in new vineyard blocks and blocks more than 25 years old.

Growers are testing blocks for red blotch virus as needed. In some cases, the infection has been found in a couple of blocks in a county, but positive cases are on the increase. A vine can be infected with the virus but foliar disease symptoms range from minimal to severe.  

Smith says vine yields are not affected.

“There are likely more vineyards infected with red blotch virus,” Smith explained. “Management may not realize it since symptoms in red varieties look similar to leafroll virus disease. In white varieties, the symptoms are very difficult to spot.”

Smith urges growers with suspect vines, given the symptoms listed above, to contact a commercial laboratory. Ask the lab for the preferred sampling procedure, gather the sample, and send it to the lab.

Besides red blotch virus, red leaves in red grapes can also be symptoms of esca (measles), Willamette mite damage, poor root health on the North Coast, and phosphorus deficiency.

Note, the veins in a red blotch-infected leaf are often red. The leaf veins in a leafroll-infected vine are green.

Symptoms of red blotch in white varieties can resemble a potassium issue.

“For most growers and vintners, red blotch virus is absolutely a non-issue, including those with vines infected with this virus.”

On the grape income front, Smith says the disease this year led to lost income for some wine grape growers in Sonoma County. On several occasions, winery representatives examined vines prior to harvest and found red blotch symptoms on red-variety leaves which were…red flags.

The wineries did not purchase the fruit.

When this occurs, Smith says the end result is reduced income. The grower has a few options for the fruit - try to find another buyer, or take the fruit to a custom crush facility for crushing, store the wine, and sell it.

“Either way, the grower will receive a lower price,” Smith said.

Red blotch virus has been found in several wine grape-producing states including New York, Washington, and Oregon but not in Arizona. The virus is in two Canadian provinces but has not been reported in Australia, South Africa, or Europe; leading competitors for U.S. wine.

California is the nation’s largest wine grape grower with more than half a million acres of wine grape vineyards. In 2011, the industry was valued at $2-plus billion.

GRBaV was first identified in the U.S.by Cornell University virologists.

Wine grape growers are learning more about red blotch virus. Last fall, UC Davis held an international conference on grape virus diseases which moved red blotch virus toward the center of growers’ radar screens. This fall, growers and wineries kept their eyes peeled for red grape varieties with red leaves.

“Industry and researchers are still learning about this disease,” Smith explained. “Research is running as fast as it can to answer the many questions that growers have.”