Managing Mildew In Grapes
February 1, 2009 Many grape growers remember 2007 as a drier-than-usual growing season with less disease pressure. But consultants like Jim MacKenzie know that drier weather can actually be more conducive for powdery mildew development in grapes. As an independent contractor for Helena Chemical Company based out of Winchester, VA, MacKenzie spends his time helping Virginia winegrape growers battle diseases like powdery and downy mildew, phomopsis, and black rot.
“Because we had a relatively dry season in 2007, many growers relaxed their spray schedules,” MacKenzie says. “But powdery mildew is still a threat in dry weather, so a lot of growers ended up having disease problems that year.”
Although 2008 started out wet, August brought more dry weather and the associated powdery mildew pressure. “Because of their powdery mildew experiences from 2007, many grape growers realized it was critical to use a protectant fungicide on the berry,” says MacKenzie.
MacKenzie’s experience illustrates the difficulty growers and crop consultants face when it comes to effectively managing powdery mildew through the varied weather each year brings. “Powdery mildew is certainly the most ubiquitous disease in grapes and a disease that needs to be managed in almost every vineyard every year,” says Wayne Wilcox, professor of plant pathology at Cornell University. The unofficial “grape guy” at Cornell University, Wilcox has spent the last 14 years studying grape diseases like powdery mildew — time that has helped him understand why powdery mildew is so challenging to control.
Powdery Mildew’s Anatomy
Unlike most fungal diseases, powdery mildew does not require free water on the plant tissue to germinate and develop. “Scientists have developed models for infection periods of most diseases based on moisture and temperature,” Wilcox says. “But because powdery mildew doesn’t need moisture to infect the plant, basically every day during the growing season is an infection period for this disease in our part of the world.”
Resistance is another challenging aspect of powdery mildew control. Evidence of resistance to the traditional powdery mildew control tools — including the sterol inhibitor (SI) and strobilurin fungicides — continues to grow.
Bob Militello, owner of Skyline Vineyards in Forestville, NY, agrees. “Managing resistance has been one of the big concerns during the years of using sterol inhibitors and other products,” he says. “We had some bad experiences years ago when we first started using sterol inhibitors. We literally wore Bayleton out and didn’t have any alternative chemistries to help us manage resistance. When more SIs came out, the problem didn’t get any better.”
Despite the continuing concern over powdery mildew, Wilcox is optimistic. “With the right tools and management practices, powdery mildew is very much a manageable disease,” he says.
Managing The Monster
When it comes to effective powdery mildew control, Wilcox’s mantra is: “During the critical period from pre-bloom through bunch closure, when clusters are highly susceptible to disease, use your best materials and your best application techniques.” This is not the time to cut corners, he adds.
One of Wilcox’s standards for powdery mildew control is Quintec fungicide from Dow AgroSciences. Containing the active ingredient quinoxyfen, Quintec is the only member of the quinoline class of chemistry. The quinoline class is distinct from other classes of chemistry used to control powdery mildew in grapes.
“Quintec is one of the first really modern compounds that specifically targets powdery mildew,” Wilcox says. “It has a fit and value as a rotational component to take the pressure off resistance developing in other materials we’re trying to use.”
Brian Olson, field scientist for Dow AgroSciences, says the efficacy of Quintec against powdery mildew stems from its ability to redistribute throughout the berry cluster. “After application, Quintec redistributes itself around the plant. What happens is quinoxyfen volatizes off the leaf, berry, or plant tissue and then is re-absorbed on adjacent untreated plant tissue,” Olson says. “It sounds like science fiction, but it’s true. This redistribution allows Quintec to provide maximum coverage and protection of grape clusters and areas of the plant not initially covered with spray.”
Wilcox recommends applying Quintec during the period from just before the start of bloom until four to six weeks after bloom, but emphasizes that Quintec should be used as part of an integrated resistance management program.
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