Pierce's disease is caused by Xylella fastidiosa bacteria, which can be spread by a variety of sharpshooter insects. The outbreak in Napa and Sonoma counties is associated with blue-green sharpshooters.
There's been a “huge increase in traditional (Pierce's disease) hotspots and in sites not normally affected,” said Rhonda Smith, UC ANR Cooperative Extension viticulture advisor in Sonoma County. Smith said the worst problems seem to be in areas with the warmest winters. The Russian River Valley and Dry Creek areas have been the hardest hit.
In Napa County, a similar patchwork of vineyards were impacted by Pierce's disease last year, said Monica Cooper, the UC ANR CE viticulture advisor in Napa County. She said there has been a "marked increase" in areas where the disease is normally seen. In areas along riverbanks where habitat restoration has taken place, the problem is diminished. In the past, non-native plants along riverbanks have acted as reservoirs for X. fastidiosa, keeping the bacteria in the area even after infected vines have been destroyed.
UC ANR CE advisor at the Central Coast, Larry Bettiga, said the area hasn't seen a corresponding increase in Pierce's disease.
"The more lush river system in Napa and Sonoma counties has plenty of areas to serve as reservoirs for both the bacteria that causes Pierce's and its vectors," Bettiga said. “We don't have blue-green sharpshooters in Monterey County.
For more information on Pierce's disease, see the UC Integrated Pest Management page on the topic.