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Leafhoppers in grapes are receiving new attention since being implicated as a vector of the emerging red blotch wine grape disease. Economic thresholds for the insect may become a thing of the past if the virus moves into Washington State vineyards and growers must shift from control to eradication.

The pest landscape for wine grapes could be changing as more is learned about a serious new disease called grapevine redleaf (or red blotch) associated virus. The disease was first described on Cabernet Sauvignon grapes in California’s Napa Valley in 2008 and has since infected many vineyards in Napa and Sonoma counties.

The disease, confirmed in both red and white varieties, has been detected in the major wine grape-producing regions of Washington, New York, other East Coast and southern states, and Canada. Symptoms in red varieties include reddening regions within leaf blades. In white cultivars, symptoms can range from subtle to obvious chlorotic regions within leaf blades.

The disease is feared because it causes inconsistent ripening and delayed maturity, and significantly reduces grape yield and quality.

Scientists are quickly getting up to speed on the new disease. Preliminary research at Washington State University has shown that the Virginia creeper leafhopper, under greenhouse conditions, can spread the disease to uninfected plants. Redleaf disease is in the geminivirus family, of which leafhoppers and whiteflies are known vectors.

Washington State has two species of leafhoppers that inhabit vineyards, according to Dr. Doug Walsh, WSU entomologist and statewide integrated pest management specialist. The grape leafhopper and Virginia creeper leafhopper are found in Washington vineyards but have not been major pests for grape growers in the past.

“But with the pending concern regarding leafhoppers and red blotch disease, this is particularly disconcerting,” Walsh said during talks of the Washington State Grape Society. Wine and juice grape growers in the state usually have few pest problems, thanks in part to the region’s cold winters and dry summers.

Before redleaf virus appeared, Walsh did not recommend spraying grapes if leafhopper populations were 18 or fewer per leaf. But the new virus could change the strategy from one of control to eradication.

Monitoring for leafhoppers will be more important in the future, he said.

Leafhoppers have a simple life cycle. Eggs are deposited on leaves. Adults and nymphs feed on leaves by puncturing leaf cells and sucking up the contents. Feeding results in a stippling of the leaves and can reduce photosynthesis.

“Adults form clouds in the vineyard,” said Walsh. “In Washington, they’re really more of a worker nuisance.”

Anagrus wasps (Anagrus epos) are efficient parasitoids of leafhoppers. In some Washington vineyards, the wasps have done a good job of reducing leafhopper numbers, he reported. Parasitized eggs can be identified by their reddish color.

Walsh’s previous research showed that looking at leaves with the naked eye is not very effective way to determine population levels. A hand lens is required.

Walsh hopes that future research at WSU will allow scientists to use DNA bar coding of leafhoppers, making identification of nymphs quick and reliable. When leafhoppers are in the nymph stage, it’s difficult to tell the species apart. Using DNA tests could also show which of the two species is prevalent in a vineyard.

“We’ll need this type of information if we determine that one species is better than the other in vectoring the disease,” he said. “It may be that we only need to be concerned about one of the two species. If we could take samples of leafhopper and run them through PCR (polymerase chain reaction) testing, we would know what species we have and how prevalent it is in the field. That will help us better control the disease.”

Walsh noted that leafhoppers are relatively easy to control and are often “collateral kill” when growers are controlling grape mealybug in vineyards to prevent the spread of grapevine leafroll disease.

Several insecticides are registered for broadcast or chemigation application (see “Leafhopper insecticides for ­Washington grapes).

Walsh said Movento is a new, systemic foliar insecticide that moves up and down the plant. “But it needs to be absorbed by the leaf and metabolized by the plant before becoming toxic to the insect,” he stressed. “Grape growers using regulated deficit irrigation should make sure they use it early in the season before grapevine leaves become old, tough, and hardened.”

Pyrethroids, a class of insecticides found in many insecticide mixes, should be avoided because they disrupt natural enemies. “My profound hope is that growers avoid pyrethroids because they kill all beneficials and oftentimes cause bigger problems with other pests, like mites, later in the season,” he said.

When using chemigation products, growers must also consider soil moisture and irrigation schedules, Walsh added. •

Leafhopper insecticides for Washington grapes

Broadcast sprays:
Lipid biosynthesis inhibitor

• Movento (spirotetramat)—spray early in the season when leaf ­tissues are soft
Neonicotinyls
• Assail (acetamiprid)
• Venom (dinotefuran)
• Actara (thiamethoxam)
• Clutch (clothianidin)
• Provado (imidacloprid)

Insect growth regulator
• Applaud (buprofezin)—target early nymphs
Pyrethroids—disruptive to natural enemies, avoid if possible
• Capture (bifenthrin)
• Danitol (fenpropathrin)
• Baythroid (cyfluthrin)
• Mustang (cypermethrin)
Chemigation products—water ­solubility impacts performance
• Venom (dinotefuran)—very water soluble
• Platnium (thiamethoxam)—­intermediate solubility
• Admire (imidacloprid)—not very water soluble