Some California wine grape growers have seen ‘red’ over the last several years in vineyards; not necessarily financial problems tied to a bank ledger, but reddening leaves on some vines which has the industry on alert.

Grapevine red blotch associated virus, or GRBaV, is the latest virus facing the wine grape industry; found in a limited number of blocks statewide with more suspected.

The major symptom of red blotch virus infection is leaves which turn red, strictly in red varietals. In white varieties, an infected vine has traditional green leaves but red blotch virus infection can exhibit symptoms similar to a potassium deficiency.

“Red blotch disease is causing a great deal of concern in the California wine grape industry,” says Rhonda Smith, University of California Cooperative Extension (UCCE) viticulture farm advisor in Sonoma County.

Red blotch virus reduces the brix level in fruit which can delay harvest. It can also affect the grape’s phenolic content which can alter juice flavor. Wineries can overcome these challenges through blending and other measures.

Smith discussed red blotch virus during the 39^th California Association of Pest Control Advisers Annual Conference in Reno, Nev., in October.

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Infected vines can also result in lower fruit prices for wine grape growers. More on this later.

Smith told the crowd, “Once a vine has red blotch virus it will always have it. Like other vine virus infections, there is no cure.”

Growers have collected cane samples from vineyard blocks for testing by commercial laboratories. Red blotch virus, Smith says, has been confirmed in the red varietals Cabernet Sauvignon, Merlot, Cabernet Franc, Malbec, Petite Sirah, Petit Verdot, and Mourvédre. The white varietals Chardonnay and White Riesling have tested positive for the virus.

All varieties appear to be susceptible to infection.

A new virus disease was first suspected in 2007 when red leaves showed up in some blocks of grapes in Napa County. Red blotch virus was not identified until 2012.

A process called Next Generation sequencing technology was used to detect the new virus in symptomatic grape vines. The complete genetic sequencing process gathers about 600 gigabytes of data from plant material and is a powerful tool to detect new viruses.

Red blotch virus is a member of the Gemini-virus family (Geminiviridae). It has been identified in vineyards along the North, Central, and South coasts, plus the Central San Joaquin Valley.

Red blotch virus cases increasing?

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.”

Insect vector a possibility

In the world of plant viruses, many viruses are vectored by an insect which raises the question, is red blotch virus transmitted by insects? Maybe yes and maybe no.

The Geminiviridae family of plant viruses is vectored by whiteflies and leafhoppers. Yet the red blotch virus is unique and different from other viruses in the family. That said, some researchers are unsure if red blotch virus belongs in the Geminiviridae family, and if that is true, whether red blotch can be transmitted by these insects.

Researchers at Washington State University have reported that the Virginia creeper leafhopper can vector red blotch in a greenhouse setting. It is unknown if the same insect could vector red blotch in a vineyard.

Meanwhile, Smith says several growers made prophylactic applications of insecticides targeting leafhoppers.

What is known, Smith emphasizes, is red blotch virus can be spread through plant propagation –taking buds from one plant for grafting to another.

Smith warned, “Don’t collect buds from one block to field bud another block unless you are 100 percent certain the budwood is clean. Test the budwood for viruses.”

The top question wine grape growers ask Smith about red blotch disease is which farming practices can best minimize the impact of the virus on the vine and fruit.

Her response is to first verify through a commercial lab that a block is positive for red blotch virus, and acknowledge the vine will never be 100 percent healthy. Then, take measures to reduce stress on the vine which are under the grower’s control, and optimize vine nutrition and soil fertility; a practice already in place by many growers.

“Good vine health is about a good irrigation strategy and a good fertility program,” Smith said.

Virus slows planting boom

Red blotch virus is also impacting wine grape plantings in California. California is in a “mini-planting boom,” Smith says.

Last winter, some growers visited nurseries which were processing their vine orders and sampled vines for testing. Growers cancelled contracts on infected vines.

Nurseries are sampling scion and rootstock increase blocks and having commercial laboratories test for red blotch virus.

“This virus took nurseries by surprise, just as it did growers,” Smith said.

Infected vines have been rogued. The process is ongoing.

“Some growers have postponed replanting blocks and planting new blocks until more is understood about how the virus is spread and how the effects of the disease can be better managed,” Smith concluded.

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