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Mother Nature threw a new curve ball at local grape growers Wednesday as thunderstorms rolled through North Coast vineyards, the latest challenge for a volatile harvest season already marked by heat spikes, humidity and cool rain sprinkles.

The thunderstorm brought 0.23 inches of rain to Santa Rosa over about two hours, according to the National Weather Service, sending growers out in the aftermath to take action against mold that could lead to mildew and botrytis, or fruit rot. They are especially concerned about chardonnay, which is ripening now and is Sonoma County’s largest varietal in terms of tonnage, representing about one-third of the overall wine grape crop.

Wednesday’s rain is the latest in a spate of extreme weather during this harvest. The most notable was a Labor Day weekend heat spike during which high temperatures reached 110 degrees and higher throughout the county. That heat wave forced many vintners to schedule picks that weekend, assuming they could find available work crews in a tight labor market. Many clusters that were left on the vines to be picked later had shriveled in the aftermath.

“With the heat, we had a 15 percent loss in weight,” said John Balletto, president of Balletto Vineyards and Winery west of Santa Rosa. “It’s huge.”

In fact, the yield of the 2017 harvest in general is expected to be less than the average crop, according to one analyst, because of the Labor Day heat spike. The 10-year average for the North Coast counties of Sonoma, Napa, Lake and Mendocino is 461,648 tons. Last year, 503,965 tons of wine grapes were harvested in the four counties.

“From what I hear over and over again, we had a good crop before the heat and now it’s coming in below average or so,” said Brian Clements, vice president at Turrentine Brokerage, a grape broker based in Novato.

While the amount of rain Wednesday is likely not enough to cause botrytis on its own, farmers were carefully checking vineyards for signs of mold after days of damp, humid weather, persistent throughout much of September and likely, according to forecasts, to stick around at least until Friday with temperatures no higher than 80 degrees.

“People are going to get a push to harvest,” said Karissa Kruse, president of the Sonoma County Winegrowers trade group, who estimated about one-third of the overall grape crop has now been picked since harvest began on Aug. 7. “You should be okay in the next 24 and 48 hours if you are picking.”

Balletto sent out tractors with large air blowers attached in order to dry out zinfandel and chardonnay grapes that are still on the vine.

“Things are stressful, to say the least,” he said, estimating that 60 percent of his crop has been picked so far. He farms more than 500 acres in the Russian River Valley.

If mold is found on grape clusters, he added, he may be forced to apply a fungicide.

“If it’s there, it will stop the spread,” Balletto said of the spraying.

The worry about chardonnay is that its grapes have thin skins and form tight clusters that are particularly vulnerable to the onset of rot. And it is reaching peak ripeness now. The white grape helped establish Sonoma County’s reputation as a premium wine region; almost 75,000 tons were picked last year at a cash value of $162 million.

North Coast Fire Insurance Claims

County: Sonoma

Homes (partial and total): 11,469

Auto: 2,659

Business: 590

Total Loss: $2.79 billion

County: Mendocino

Homes (partial and total): 447

Auto: 112

Business: 11

Total Loss: $118 million

County: Lake

Homes (partial and total): 250

Auto: 29

Business: 3

Total Loss: $51 million

County: Napa

Homes (partial and total): 1,613

Auto: 239

Business: 104

Total Loss: $273 million

Source: State Insurance Department

Previous Costliest US Wildland Fires (insured loss)

Fire: Oakland Hills fire, California

Year: 1991

Cost: $1.7 billion

Fire: Witch fire, California

Year: 2007

Cost: $1.3 billion

Fire: Cedar fire, California

Year: 2003

Cost: $1 billion

Fire: Old fire, California

Year: 2003

Cost: $975 million

Fire: Great Smoky Mountains fire, Tennessee

Year: 2016

Cost: $938 million

Fire: Valley fire, California

Year: 2015

Cost: $921 million


Read all of the PD's fire coverage here

In contrast, the threat of rot for Bordeaux varietals such as merlot and cabernet sauvignon is not as great, as those grapes have thicker skins and ripen later in the season, which typically ends around Halloween. And about 80 percent of the pinot noir crop — the county’s most expensive varietal at $3,681 per average ton in 2016 and also thin-skinned — has already been picked, Kruse said.

At Chalk Hill Estate, owner Bill Foley said he encountered chardonnay grapes that were shriveling or “raisining” on the vine before the thunderstorm struck. His workers started harvesting chardonnay on Tuesday, an effort that will be sped up because of concern about botrytis.

“We have 130 acres of chardonnay on this property,” said Foley, proprietor of Foley Family Wines. “We better get it in.”

Foley said he hoped the 2017 harvest would not resemble that of 2011, when continuing rains and cool weather resulted in a crop with very low yield for the North Coast. Foley noted he lost about 40 percent of his chardonnay crop in 2011. “Chardonnay, that’s Chalk Hill,” he said.

Reports of botrytis are not widespread because growers have been diligent, Kruse said, but there certainly are pockets spread across the county.

“They are finding one bunch on 12 to 15 vines,” she said.

To prevent further damage, growers can spray fungicide, though they then have to wait seven to 10 days for the residue to dissipate before they can harvest, Kruse said. They can also remove more leaves from the vine canopies, but that’s risky because another heat spike could shrivel the unshaded grapes, making them worthless.

Others may opt to leave the lesser quality fruit on the vine, or throw it away as it goes through the sorting line at the winery, Kruse said.

“It’s been a little unpredictable harvest,” she said. “It’s been one of the most varied from a weather perspective.”

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

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