Grapes become less susceptible to powdery mildew as berries soften and accumulate sugar and it’s tempting to let down your guard then and dismiss the possibility of further threat from the fungal disease after veraison.
However, the fungus thrives in temperatures of 70 to 85 degrees, regardless of berry maturity, and those temperatures can occur season-long.
A powdery mildew epidemic can begin with just six hours of such temperatures inside the canopy for three days in a row. As many California grape grower experienced last year, those conditions can happen after veraison. Consequently, unrelenting powdery mildew pressure after veraison in 2011 prompted growers in some areas of the state to continue treating their vineyards with fungicides until harvest.
The key to season-long control of powdery mildew is to reduce early-season inoculum and subsequent infection. To be effective, treatment must begin promptly and be repeated at appropriate intervals. Timing of the first treatment depends on the fungicide used and growth stage. After that, frequency of treatment depends on the choice of fungicide and weather conditions. You can determine how often you need to spray to protect vines by monitoring the vineyard for signs of the disease and using the powdery mildew Risk Assessment Index (RAI) developed by the University of California, Davis to assess the disease risk.
“The best way to control powdery mildew is to be very proactive and diligent in your efforts to prevent a full-blown infestation in your vineyards,” says PCA Sam Turner, whose company, T&M Ag Services, Calistoga, Calif., manages over 800 acres of wine grapes in the Napa Valley for more than 20 clients.
“If you can keep your vineyard clean of powdery mildew starting in early in the season, you can significantly reduce your chance of getting an outbreak of powdery mildew in June or July,” he says. “Although the likelihood of the disease infecting clusters is much less after veraison, failure to control powdery mildew earlier in the year can leave a lot of inoculum in the vineyard. It will get into the wood, where it will overwinter and increase powdery mildew pressure the following year.”
Early-season control of powdery mildew is easier than trying to contain it later. Getting sprays to penetrate the canopy and protect berries is easier early when there is less foliage. It becomes more difficult as the vines put on more leaves.
As harvest gets closer, the choices of fungicides suitable for treating powdery mildew may be limited due to concerns about adverse impacts of some materials on yeast activity and flavor during the winemaking process.
Powdery mildew can also lead to other disease damage. It can leave vineyards vulnerable to late-season bunch rot, Turner notes. “If a powdery mildew infection is severe enough, it can prevent the berries from ripening and cause them to split. And that can open the door for rot organisms.”
Another reason to stay on top of powdery mildew threats throughout the year is that an outbreak late in the season can be more costly than an early infestation. Yields will be reduced on vines infected before berry set. Powdery mildew also imparts an off flavor in wine, and a grower risks getting his grapes rejected at the winery if the disease is prevalent.
Most powdery mildew fungicides are preventive rather than curative, says Lowell Zelinski, whose Paso Robles, Calif., firm, Precision Ag Consulting, provides vineyard management, irrigation scheduling and fertilization recommendations services for wine grape producers.
“If you don’t treat the disease early in the season, the disease can spread quickly,” he says. “Getting the resulting infection under control is much more expensive and labor intensive.”
A fungicide solution mixed with 10 gallons of water per acre may have to be increased to several hundred gallons per acre to effectively deal with powdery mildew, he says.
“With a preventive treatment, you’re covering the berries with a light coating of fungicide. But, when you go in to cure the disease you have to actually wash the spores off, and that takes a lot of tankloads of solution. Even if you cure that infection, another one can start as long conditions favor spore growth. So, you’re back to a preventive treatment program.”
Fighting a flare-up of powdery mildew may also require stronger medicine than prevention. University of California pest management guidelines for powdery mildew control call for discontinuing the use of soft chemistry products when disease pressure is high because, by themselves, they will not provide adequate control. Under these guidelines, if eradication is necessary, a light summer oil may be used anytime in the season, as long as there is no sulfur residue present. That would be at least two weeks after a sulfur treatment
Minimizing the impact of powdery mildew requires prompt treatment and applying continual pressure on the disease-causing spores to keep them in check. It means going the extra mile — but, it can pay off with season-long protection of your crop and your profits.