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.