(Wines&Vines) - Grapegrowers in Napa and Sonoma counties experienced an explosion of Pierce’s disease (PD) this year, and it’s attributed to the native blue-green sharpshooter, not the threatening glassy-winged sharpshooter that so far has been excluded from the counties by rigid quarantines.

Researchers are hoping to uncover the reason behind the spike in PD, but many people are speculating that the mild weather we’ve had the past two winters may be a culprit.

Meanwhile, a new method of addressing the disease developed in Texas, where PD may have originated and still is rampant, shows promise.

Sonoma County hard hit

Rhonda Smith, the viticultural advisor for University of California Cooperative Extension in Sonoma County, said, “A huge increase in Pierce’s disease occurred in vineyards that are traditional hot spots as well as in sites not normally affected, pointing to an epidemic associated with warm winters.”

She said that growers removed tens of thousands of dead vines in the spring and replanted, but the demand for plants outstripped supply.

Smith added that the incidence of PD has been slowly increasing during the past few years, and a larger increase was seen in 2015.

Doug McIlroy of Rodney Strong Vineyards is on the board of the Sonoma County Winegrape Commission. His family has grown grapes in the Russian River for more than 40 years.

“I’ve never seen PD like this before,” he exclaimed, noting that outbreaks seem to occur in cycles. “It seems to peak around warmer El Niño years.” He thinks cold weather affect the bacteria.

McIlroy said the grapevine illness seems especially damaging to early leafing varieties like the Chardonnay and Pinot Noir that Sonoma County is best known for. He finds it especially prevalent in the Russian River Valley southwest of Healdsburg, Calif., and in Dry Creek Valley, both relatively narrow valleys with intensive riparian growth.