(Wines&Vines) - An unlikely program attracted a standing-only crowd to Napa’s Century Theater recently: the current outbreak of Pierce’s disease in Napa Valley and other parts of the North Coast of California.

For the grape growers who attended the unusual open meeting of the Napa Valley Vineyard Technical group, the news wasn’t good. “I can’t give you any simple answer to controlling the disease,” admitted Matt Daugherty of the University of California, Riverside.

Sandy Purcell, the renowned and now-retired expert on the subject, added, “Expect a lower late of recovery this year due to the warm winter that’s accompanying the welcome rain.”

The meeting was held in the movie multiplex—complete with popcorn and sodas for attendees—but Vit Tech returns Feb. 11 to its former home at Copia, which has been acquired by the Culinary Institute of America.

Normally, you have to be a member of the Vit Tech group to attend meetings, but Napa’s viticultural farm advisor Monica Cooper said, “I’ve had so many questions that I opened this to the public,” though she encouraged attendees to join the group.

The meeting started with a survey of attendees about their experiences with PD as part of a three-year study being conducted by Rodrigo Almeida and Adam Zeilinger of the Department of Environmental Science, Policy and Management at the University of California, Berkeley. They didn’t disclose results yet for fear of compromising other surveys.

What do we know about the outbreak?

Almeida then outlined what is known about the current outbreak, notably the impact of weather on the disease.

Pierce’s disease, which is caused by Xylella fastidiosa bacteria, clogs the xylem in vines, eventually killing them. It colonizes a wide range of host plants, usually without causing disease.

PD is found in the Americas, Taiwan, Italy, France and Iran and has been causing disease in grapevines at least since the 1880s.

The major crops affected include grape, citrus, alfalfa, peach, almond, plum, coffee, etc.

PD is spread only by sap-feeding insects, namely the glassy-winged sharpshooter and the blue-green sharpshooter (BGSS), as well as some spittlebugs. Nymphs and adults transmit the disease and there is no latent period, but it is not passed along to progeny.

Surprisingly, the impact is not primarily from the population of blue-green sharpshooters that transmit Xylella, but on when they are active and how quickly vines recover. That appears to be impacted by how warm the winter is.

These local sharpshooters—feared glassy-winged sharpshooters (GWSS) aren’t a factor as they aren’t yet found in Napa County due to quarantines and monitoring—typically live in riparian corridors.

Ironically, Almeida says that GWSS is a poor vector (per individual insect) of X. fastidiosa from/to grapevines. The native BGSS is an efficient vector.

The insects hatch in the spring, then spread into nearby vineyards. They aren’t strong flyers, however, so they tend to mostly infest areas near their homes in river and stream banks. However, the later in the summer they infect vines, the less likely the vines are to be damaged.

It turns out that infected vines can recover over the winter, but the warmer the weather, the less likely they are to recover.

This phenomenon may be partly related to pruning infected parts of the vine, but may have other causes.

How to manage Pierce’s disease?

Matt Daugherty from the Department of Entomology at UC Riverside discussed specific management strategies such as barrier treatments and severe pruning, and the impacts of “cold-curing” on disease epidemics.

He covered four techniques:

1. Vector control

2. Eliminating inoculum supply

3. Host resistance

4. Chemical control of sharpshooters

Insecticides for sharpshooter control include conventional systemics and foliars, plus some organics.