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Having ramped up hard to get through a triple-digit heat spike over the weekend, North Coast grape growers are now navigating a new challenge from Mother Nature — muggy, humid weather conditions this week that bring the threat of mildew and botrytis, or fruit rot.

While growers maintain the risk is minimal for a significant outbreak, they are closely watching the weather forecast, which the National Weather Service said includes a 20 percent chance of rain showers Thursday.

“The spores are there. You have the right condition for humidity and temperature, the spores will drop and you have botrytis,” said Duff Bevill, of Bevill Vineyard Management in Healdsburg, who farms about 1,000 acres around the Alexander Valley.

While all varietals can be susceptible to rot, most growers keep close tabs on chardonnay because their thin skin and tight clusters make them particularly vulnerable to the onset of rot, said Karissa Kruse, president of the Sonoma County Winegrowers trade group. They also are harvested later in the season than some other varietals, which exposes them for longer to potential adverse weather events.

“The biggest trigger would be a significant amount of rain — at least a half inch of precipitation, if not more,” said Steve Dutton, who with his brother, Joe, farms about 1,200 acres around the Russian River Valley.

“A few little sprinkles isn’t going to do it,” Dutton said of the threat.

After checking her Russian River Valley and Carneros vineyards, Alison Crowe, director of winemaking for Napa-based Plata Wine Partners, said the conditions were not present on Tuesday to overly worry about rot, especially as high heat helps eradicate it.

“The grape berry surface needs to be wet, wet, wet for consecutive periods,” Crowe said, in order for rot to take hold.

The weather service predicts that by the weekend, temperatures should return to more normal for this time of year. Healdsburg, for example, should reach a high of 87 on Saturday and 88 on Sunday. “The heat really puts a stop to the microbiological bloom out there,” Crowe said.

Growers have picked about 15 to 20 percent of the overall grape crop so far in Sonoma County, Kruse said. Fruit for sparkling wine, generally picked earliest, likely will be all harvested by week’s end.

Many wineries have sped up their harvest of pinot noir and chardonnay as the heat wave accelerated ripening, she said, although Bordeaux varietals like cabernet sauvignon and merlot will likely not be picked until toward the end of September. Harvest started on Aug. 7.

This week’s conditions are a zig-zag from last week’s, when winemakers frantically raced to pick fast-ripening grapes that were responding to soaring temperatures. Temperatures over much of the Labor Day weekend hit highs of 110 degrees and more, according to the weather service.

“We are just in ‘go’ mode,” Kruse said.

Winemakers were scheduling picks as much as five days in advance to ensure they could get their grapes harvested before the heat caused any damage, Bevill said. Flavor balances can be upended and, in extreme conditions, grapes can shrivel or “raisin.”

“It was pretty stressful with people calling and waiting to schedule,” he said. The heat did not cause any widespread damage to his crop, Bevill said, though certain areas that were not readily accessible to irrigation had lower yields than normal.

Vineyard managers also scheduled their work crews to start around midnight, so their employees could work through the night to finish their picks before the daytime heat would become unbearable.

“The big thing for us is our guys,” Dutton said. “We wanted to make sure they were done by 10:30 a.m. at the latest.”

You can reach Staff Writer Bill Swindell at 707-521-5223 or bill.swindell@pressdemocrat.com.

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