Researchers offer suggestions to stop leafroll virus

The best strategy for winegrape growers who detect leafroll virus in their vineyards, according to leading researchers from around the world, is to rogue early and often to prevent spread of the disease, which, left unchecked, will destroy entire blocks.

There are vector controls, including some low-impact materials, but none of them have the knockdown capacity needed to prevent wider transmission of the disease by the fast-multiplying vine mealybug that carries the pathogen from vine to vine.

"We've moved away from the toxic contact materials to the systemic materials," said Kent Daane, University of California Cooperative Extension specialist in biological control. "The vine mealybugs may die in three days, but they have already done their job of vectoring the leafroll virus."

He reported progress in the development of environmentally friendly controls that work once reservoirs of the virus have been reduced to very low levels throughout an entire region.

"There are two firms with pheromone dispenser, Pacific Bio Control and Sutera," Daane said. "Lacewings, mealybug destroyers and some ladybug beetles help, but none of these will get the mealybug population down to zero. We are never going to get the vector populations down low enough; we also have to rogue."

Daane made his remarks to the growers and vineyard managers who filled to capacity the largest conference room at the San Joaquin County Agricultural Center for the Lodi Winegrape Commission Mealybug & Virus Outreach Meeting.

The meeting was part of an ongoing effort to research and control the spread of leafroll virus, cosponsored by the Winegrape Commission, the California Department of Food and Agriculture, and the American Vineyard Foundation.

Researchers from around the country, and around the world, presented their advice on managing the virus, which has spread to commercial vineyards in every area of the state since the vine mealybug was first reported in Coachella a quarter century ago.

"The principle is to get into your vineyard as soon as possible and rogue," said Gerhard Pietersen, professor at the University of Stellenbosch in South Africa. "It is the least painful way to deal with it."

The South Africans became the experts on managing leafroll virus in vineyards because they had to.

"Our vineyards have to be replaced every 20 to 25 years because of leafroll virus; it has tremendous economic consequences," Pietersen said. "Leafroll virus is the most important disease in South African vineyards. The virus is so widespread, and the vine mealybug has been there since the 1930s, so we had to study leafroll."

A strategy that has proven effective under even these conditions of extreme disease pressure is to go the extra mile in identifying newly planted vines with the virus and rogue them diligently.

"One technique is to plant new vines, treat them with a systemic material to control the mealybugs to prevent new infection, and then remove the vines that show symptoms," Pietersen said.

While starting with clean stock is essential, the virus can be in new plants at levels so low as to be undetectable by even the most rigorous nursery protocols.

Established vines must also be monitored and diseased plants removed, down to the last bit of the roots, to prevent the slow but inexorable spread of leafroll throughout the vineyard and beyond.

"Once a vine is infected in the vineyard there is no cure," said Mark Fuchs, professor at Cornell University. "The virus will remain until the vine dies or is removed."

The virus, fortunately, has vulnerabilities that can be exploited in developing a management plan.

The pathogen can only survive on vitis species, which means farmers do not have to worry about reservoirs in weeds or other crop plants.

And the disease can only infect new vines with the help of a limited number of insect vectors.

"It cannot penetrate the vine without a vector," Fuchs said. "Leafroll virus is transmitted by mealybugs and soft scale. It is not transmitted by any other vector. Hedging, pruning, harvesting or suckering do not move the virus to new vines, although they can move the vectors to new areas."

The virus also does not multiply inside these important vectors, and is even lost entirely when they molt.

Because the pathogen can be moved easily, if slowly, from one vineyard to the next, strategies to remove diseased vines and manage vine mealybugs are best implemented over a large area.

"Areawide roguing has been incredibly effective; leafroll can be controlled," Pietersen said. "Once the virus has been reduced, you can tolerate much higher mealybug populations and then you can go into biological control."

An areawide approach is also most effective in a low-impact program that combines mating disruption and biological controls for the invasive vine mealybug, which is the most important of the mealybug vectors because it has six or seven generations a year.

"The larger the area, the more effective mating disruption will be," Daane said. "I like mating disruption, but it's got to be in a large area. I'd like to see areawide mating disruption."

Once leafroll virus is in the neighborhood, there is tremendous economic incentive for growers to join in an areawide management program.

"The economic impact for red blotch can be $610 to $1,100 per acre," Fuchs said, "but for leafroll it can be $485 to $3,660."

(Bob Johnson is a reporter in Davis. He may be contacted at bjohn11135@aol.com.)