"If you don't find mildew in your vineyards, you haven't looked hard enough," said Glenn McGourty, a viticulture advisor for the University of California Cooperative Extension (UCCE) for Mendocino County. After years of drought conditions, California vintners are enjoying wetter conditions this year. But that means vineyards are being plagued with a new problem: mildew, including downy mildew, a form rarely seen in the Golden State. From cooler coastal regions experiencing an increase in a damp marine layer to Central Valley vineyards with more moisture than usual due to a very wet winter, growers and vineyard consultants alike are on the lookout for mildew.
Mark Battany, UCCE viticulture advisor for San Luis Obispo and northern Santa Barbara counties, says that cooler coastal areas in those counties have been ripe for powdery mildew this year. "We've also seen some limited downy mildew, a European import, in a few locations this season," he said. "Quite rare for California."
Battany adds that warm and wet spring weather, following the earlier wet winter, has created perfect conditions for downy mildew in both counties. Both types of mildew are capable of damaging plants and berries and impacting quality and quantity of wine.
Lawrence Sterling, director of operations at Iron Horse Vineyards in Sonoma County's Green Valley, says the conditions for mildew are simple: "It loves moisture and lower temperatures, temps between 70° F to 85° F, and it stops when grapes reach about 18° Brix." His team has been busy looking for it this year. "The Russian River Valley and Green Valley are powdery mildew heaven because of the marine layer, which brings the moisture and the lower temperatures that mildew loves."
Sterling and other experts agree that prevention and early detection are the keys to any type of mildew management, with aggressive spraying and canopy management techniques leading the fight. "In a year like this, we need applications even before bloom to keep mildew under control," said McGourty. "If you don't get on top of it from the start, you're gonna have trouble."
Sterling says that at times he's had vineyard crews hand washing bunches of infected grapes. They've even dropped 5 percent to 10 percent of the crop in difficult years.
Dana Merrill, owner of Mesa Vineyard Management in San Luis Obispo, adds that growers can't just use sprays. "Canopy management is a critical component in the mildew battle as well," he said. "Keeping canopies open to facilitate air movement and spray coverage is critically important and increasingly these days that means mechanical leaf removal—certainly in the cooler regions."
Careful canopy management has the added advantage of appealing to growers and consumers who prefer to avoid spraying the vines and grapes. Sulfur sprays can be used by organic growers as well as conventional, but may affect taste if used after fruit set, so growers often avoid an excess of sulfur. But there are other fungicides, some even approved for use on organic crops, that can help control mildew outbreaks.
In April of this year more than 200 vineyard owners and managers concerned about overuse of sprays attended a Sustainable Winegrowing Day sponsored by the Sonoma County Winegrowers at Shone Farm in Forestville to learn about new technologies that could help them manage mildew while limiting spraying.
Thankfully, the threat may be passing as summer heats up and temperatures rise around the state. Mildew cannot survive in prolonged higher temperatures.