Grapevine leafroll virus, able to spread through vineyards from infected propagation material, vectors, human activity, and root grafting, is a growing problem in Washington State.
Several years ago, based on a statewide survey, industry experts estimated that 10 percent of the state’s wine grape acreage was infected. Today, anecdotal evidence suggests the disease is more widespread than previously estimated, says a Washington State University virologist.
Dr. Naidu Rayapati, WSU grape virologist, believes that leafroll virus is a threat to the sustainability of Washington’s wine grape industry. Left unchecked, the virus reduces the lifespan of a vineyard by delaying ripening, lowering the quality of fruit and wine, and robbing yields and profitability. Yield losses of up to 50 percent have been reported. In a study comparing healthy and infected Merlot vines growing side by side, Rayapati saw yield reductions of 30 percent, with sugars reduced by 6 percent.
Leafroll virus can’t spread in the vineyard by physical contact—one vine touching another—he emphasized. "But it does spread by vegetative propagation and by grafting infected cuttings," he said during the Washington State Grape Society meeting in November. Vectors, including mealybug and scale, can also spread the disease.
The virus has a wide variation of symptoms, he said, noting that red wine grape cultivars usually show coloration symptoms in leaves, with green or yellow veins and red, purple, yellow or white discoloration on the leaves between the veins. But discoloration does not occur in white wine cultivars.
"White grape varieties will show a downward rolling of the leaves, which is where the name comes from," he said. "It’s hard to find the virus just by looking at the leaf symptoms. Sometimes you’ll see typical symptoms, but not always. You have to use diagnostic tools to make sure the plant material is clean."
The disease is also tricky to diagnose because there are many strains of the virus. Ten different strains have been identified, making grape leafroll one of the largest and most complex groups of plant viruses, according to Rayapati. Of the six grape leafroll associated virus strains that have been detected in Washington, GLRaV-3 is the most common.
Vineyards are often infected with more than one strain. WSU scientists found that 56 percent of vineyards that were part of a reconnaissance study in Washington were infected with a single strain, while 44 percent had multiple infections coming from one to three different leafroll strains.
It is important that testing looks for all strains of the disease, Rayapati stressed. "You need to test for all of the types and not just one of the most common associated strains," he said.
Infected vines often show up in a vineyard as clustering within a row, he said. The spread of the virus can occur through several means—including mealybug vectors, root grafting, pruning, hand harvesting, and mechanical harvesting—that move the vector up and down rows and into new areas.
If a new planting is located near an infected vineyard, clustering is usually noticed in the rows of the new planting that are closest to the infection. Generally, leafroll virus gains a foothold in the vineyard by first infecting vines at the end of the rows, and then moving inward.
Rayapati, who is studying the role that root grafting may play in the vineyard, found that in a greenhouse, it is possible to spread the virus from root grafting. He has seen the virus spread from an infected vine to a healthy vine in Cabernet Sauvignon and Chardonnay sharing the same pot. "But does this happen under normal conditions?" he asked. "It is possible, but we don’t really have definitive answers."
Concord vineyards could also play a part in the spread of the virus. Rayapati reports that they found GLRaV-2 and 3 in a Concord vineyard that did not show typical leafroll symptoms. "We don’t have extensive data that shows that there is spread from Concord to wine grapes," he said. But they did find several vines in a wine grape vineyard that tested positive for leafroll that were near an infected Concord planting.
"The location of the infected wine grapevines is probably suggesting that the disease might be coming from the Concords," he said, adding that mealybug could be involved in the spread.
Removal of infected vines is the only treatment for leafroll disease. Using virus-free plant material is the best prevention, he said, adding that growers need to know which virus and strains have been tested for, when planting or grafting.
"Leafroll disease is not a simple problem," he said. "It has complex symptoms and is a complex virus. Though we have detected only six of the ten strains in Washington, it’s possible that we have all ten here. We have only a few answers, but we have many questions."