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Early detection key to disease management, researchers say

‘Rogueing the infected vines and replanting with healthy cuttings can help eliminate the problem,’ scientist says.

When it comes to the diseases brought on by pests in the West such as the grapevine mealybug, the is there is little in the way of good news.

“We’re beginning to see signs of a mealybug population in Washington State vineyards that is developing a tolerance to preferred insecticides like imidacloprid (from the neonicotinoid group toxic to sucking insects) and by extrapolation, if they’re becoming resistant to that one, they’re probably going to have cross resistance with other insecticides registered for wine grapes.”

The bearer of that bad news was Washington State University (WSU) entomologist Dr. Doug Walsh speaking to the recent WAVEx-Grape Virus Detection seminar for grape growers and winemakers at WSU’s Prosser Research Station.

“That’s been the cheap insurance policy up till now,” he told the group.  “I’d say 60% or more of the acreage in Washington has been treated annually and its effectiveness is beginning to wane.”

To which he noted, “We conduct transmission bioassays in laboratory environments where newly-hatched mealy bug crawlers are infected with leaf roll and then transferred to potted vines that are free of any virus. The bad news is that 10% of the time, that single infected bug will transmit the disease — and there’s a whole lot of mealy bugs out there in the environment.”

So, what to do in the way of pest management? “We want to develop a threshold in integrated pest management where if an insect population remains below a specific density in an agricultural ecosystem, you don’t treat, you don’t spray. We took some of the traditional systemic insecticides and treated those vines at twice the maximum label rate — a toxic dose — and it seemingly made no difference. In just a short period of time, the infected mealy bugs feeding on the clean vines was enough to vector the disease.”

Reducing insecticides

All that said, he made his point to the cultivators: “Growers need to continue to manage leaf roll disease with insecticides, but they don’t need to go overboard in an irrational, exuberant, mealy bug management program. I propose a more rational approach that includes imidacloprid or other neonicotinoids via chemigation at least every other year along with systemic insecticides that provide effective mealy bug control during the vigorous early summer growing period.”

In a WSU press release, Walsh and plant pathologist Naidu Rayapati note that “There is no cure once a grapevine is infected which has historically meant the first line of defense is the use of clean plant material for vineyards. However, in recent years, young vineyards planted with clean stock have shown virus symptoms within a short period. Our goal is to bring scientists and industry members together to raise awareness about the value of research among growers and wineries.”

Rayapati focused much of his presentation on diseases themselves — fungal, bacterial, or human — and their causal agents. “Once we know whether it’s a known virus or a new one we can more quickly understand its epidemiology and offer control measures. Rapid and accurate detection and diagnosis is a fundamental for coming up with disease management options.”

Like other West Coast growing locations, leaf roll and red blotch are two of the bad guys. “Rogueing the infected vines and replanting with healthy cuttings can help eliminate the problem in a specific vineyard block as a low-cost tool to manage red blotch,” he said. “Leaf roll requires a more integrated approach in vector control.”

Because the Washington wine industry is a youthful one, “We can learn lessons from other growers like those in California or Europe where these problems have already become a challenge to control. We can learn to be more proactive and not make mistakes of the past.

“Multidisciplinary research is often the most effective approach to develop solutions to complex problems in agricultural production systems and our objective is to work with growers in a participatory approach to provide them with the best management practices to deal with diseases.”

For more news on pests, disease management and other issues affecting vineyards, subscribe to the bi-monthly newsletter The Grape Line.

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