Mildew pops up in Sonoma County vineyards

The temperate weather Sonoma County has experienced this summer may be great for recreational activities and outdoor dining, but it is keeping vineyard managers on guard because mildew is threatening the county’s $600 million grape crop.

Some farmers in the county have spotted signs of grape clusters and leaves with powdery mildew, a fungus that can dim quality and lessen quantity, if left unchecked.

“It’s definitely more than normal,” said Tom Gore, director of vineyards for Sonoma and Mendocino counties at Constellation Brands Inc. “It’s not catastrophic by any means. … We have a little bit in quite a few places.”

The outbreak has been triggered by mild summer temperatures since May’s fruit set, the stage when the flowers are transformed into tiny grapes. The ideal temperature for such fungus growth is between 70 and 85 degrees; temperatures above 95 degrees for 12 hours can halt its growth, a common index developed by UC Davis scientists notes.

Twelve areas in Sonoma County were under the severe stress category for powdery mildew Thursday, according to a daily alert from Western Weather Group in Chico.

One reading in Forestville and another in Kenwood on Thursday reported more than nine hours of temperatures between 70 and 85 degrees, conditions that allow mildew to spread. Western Weather predicted that the stress “should increase or stay severe” through next week because of slightly cooler daytime temperatures than normal.

While all parts of the county are susceptible to the threat of mildew, areas such as the Sonoma Coast and the Russian River Valley are at greater risk because of the prevalence of the marine layer, which limits the temperature swings seen in other areas. Chardonnay grapes are the likeliest to have such fungus, said Mark Greenspan, a vineyard consultant with Advanced Viticulture in Windsor.

“If you get a real bad infection, it essentially puckers the fruit,” Greenspan said. “A minor infection can be washed.”

Bacchus Vineyard Management has had seven people this summer out scouting the 1,000 acres it oversees mostly in Sonoma County, including properties such as J Vineyards, owner Glenn Alexander said.

“I would say we have had a consistent mildew pressure this year,” said Alexander, whose crews sprayed fungicide on some areas last week. “You don’t get a break.”

Treatment against the fungus continues until the fruit reaches a certain sugar level, typically at about the halfway point to harvest. But Alexander noted that veraison - the French term for the onset of the visual signs of ripening in grapes - may be longer this year because of a longer bloom time earlier this year. Harvest appears to be on track to start in mid-August, farmers said, though fruit destined for sparkling wines could come as early as late July.

The main tool to combat powdery mildew is attentive and aggressive canopy management, which allows more air and light to reach the grapes.

“Ultraviolet light destroys mildew,” Greenspan said.

Sulfur traditionally has been used to get rid of the fungus once discovered, though winemakers do not like to use it after fruit set because they are worried it could affect the taste. Other fungicides, however, also are available, including those used for organic grapes, Greenspan said.

In severe cases, Gore said, workers will likely trim off the cluster to prevent further spreading. “Once you have a mildew outbreak … it’s kind of an uphill battle to remove,” he said.

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