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Wilcox details diseases most worrisome for fruit growers

By RICHARD SKELLY
AFP Correspondent

MONROE TOWNSHIP, N.J. (May 9, 2017) — Grape growers received advice on  battling diseases from retiring Cornell University plant pathologist, Wayne Wilcox, during this year’s Grape Expectations Symposium. Wilcox, has spent much of his career studying diseases of apples and apple trees as well as grapes grown in New York.
“This is the best part of my job,” Wilcox related at the outset of his talk to an audience of about 100 winery owners and grape growers at Forsgate Country Club Wilcox presented various examples of diseases affecting grapes in a slide show presentation and pointed out that the climate is different in New York than it is in southern New Jersey.
Wilcox addressed downy mildew, a common disease in vineyards first.
“Traditional fungicides for downy mildew included copper, they discovered this in Europe a long time ago,” Wilcox said “But when it’s raining like hell, the fungicides stop working,” he said, noting Ridomil Gold is the best fungicide he’s found in the Finger Lakes region for control of downy mildew.
“The problem with Ridomil is there’s a high risk of resistance. Our growers will use it two times in three years, ‘cause when they need the nuclear option, that’s what they use, but it’s expensive,” Wilcox said. With merlot grapes in New York, he said the first signs of infection usually happen about two to three weeks before bloom, and downy mildew can snowball in as little as four days.
“Last year there was a case of resistance that was confirmed in Virginia. Was that the tip of the iceberg? Is it much wider spread? We don’t know yet. With any of our newer materials we’re going to have to limit their use to prevent developing resistance,” he said. Wilcox added that new fungicides are being created that work against black rot, downy mildew and powdery mildew.
“There are a lot of new combination products coming out and sometimes they’re combined to help guard against resistance,” he said, “but they’re always combined because the company has both of them and they want to sell both of them to you.”
“Zampro is a combination product designed not for broader activity but to have resistance management and the other part of Zampro is new chemistry behind it that’s downy mildew specific,” he said. “It appears to provide some significant post-infection protection activity and this combination is one we’ve had excellent results with in our trials.”
Wilcox added the 2014 growing season was “hellacious” for downy mildew in New York, due to so much rain that summer, as he showed slides of grape vines afflicted with the fungus.
“If I could work with a biological control product that actually worked, nothing would make me happier,” he said, adding, “downy mildew is not something we want to be wrong about.”
The downy mildew fungus grows in old, dead wood, so one thing grape growers can do is prune out dead wood from vines, he said, urging grape growers, “use a real fungicide and use them on a limited basis; if you use fungicides to death then they don’t work.”
In addressing powdery mildew, he said, like downy mildew, humidity and rain is critical for the growth of this fungus.
“Powdery mildew doesn’t need a lot of rain to cause infection, the fungus can pull all the water it needs out of water vapor in the air,” he said, “but once the temperatures get to be about 95 degrees, it doesn’t grow as well.”  Leaf temperature has a lot to do with the spread of powdery mildew, he added, so temperatures in the 90s are often lethal to the fungus.
“The ways to control it include careful canopy management and ventilation,” he said. “IPM techniques can help control the disease and all good growers do that by definition anyway; they don’t just rely on their fungicides.”
“Black rot is also an issue for warmer climates,” Wilcox said. “You get it worse here than we do in New York State.”
He said decades ago when he was working with apple growers on tree diseases, and said, “I thought if you would spray pre-bloom, it would control black rot, from working with apples,” but black rot is “a weak pathogen, not a very good pathogen, it likes things that are starting to break down.
“So as the berries are ripening they’re really susceptible to infection from black rot.”