Sonoma, Napa grape growers learn the latest on fighting powdery mildew

One of the biggest threats to vineyards local grape growers face is powdery mildew, a fungus that can ruin their crop if left unchecked.

But spraying can be problematic, so the more than 200 vineyard owners and managers who attended the annual Sustainable Winegrowing Day at Forestville’s Shone Farm Tuesday were eager to learn of new technologies to help keep the fungus at bay and limit spraying at the same time.

The event was sponsored by the Sonoma County Winegrowers, the trade group that represents local vineyard owners and managers.

“It’s huge. Of all the things a grower faces in a year, powdery mildew is the most significant,” said Laura Breyer of Breyer’s Vineyard IPM Services in Windsor.

One new development is a device that can trap spores of the powdery mildew and use DNA sequencing to determine if there is in fact an active fungus present that could diminish the crop, said Bryan Rahn, a soil scientist at Coastal Viticulture Consultants in Angwin.

Typically, Rahn said, grape growers “are spraying to control a pathogen” that may not be present.

The device, which costs $1,600 a year, serves as an early warning detection system that can alert vineyard personnel to they need to spray immediately. It also can help space out any subsequent treatments throughout the rest of the growing season to prevent over spraying, Rahn said.

“We have about 80 of (the devices) out this year. We got quite a collection in Napa. We are getting a bigger collection in Sonoma,” he said.

The Sonoma Coast and the Russian River Valley are typically the most susceptible to powdery mildew because of the foggy marine layer that lingers over both regions, according to analysts. Temperatures above 95 degrees for 12 hours can halt mildew growth, according to ?UC Davis.

Speakers at the event also emphasized that such new technologies can help reduce the need to spray insecticides, fungicides and herbicides to protect their crops. The issue has taken on more prominence in the industry with criticism of the weed killer known as Roundup, which has been deemed safe by the EPA if used according to label directions but has been opposed by some health advocacy groups.

Sulfur is a popular option among local farmers to combat powdery mildew, though copper-based fungicides and certain plant-based oils also are used. Sulfur was the most widely used pesticide in Sonoma County in 2014, according to a state database, with 1.4 million pounds sprayed on crops.

“Growers used to just start on a calendar date and go out with the strongest material that they had and keep spraying until they were done,” Breyer said. “That’s not the way we farm anymore.”

You can reach Staff Writer Bill Swindell at 707-521-5223.