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 39th 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.