'Huge' Outbreak of Pierce's Disease

Mild weather could be contributing factor in Napa and Sonoma counties

by Paul Franson
Grapegrowers in California’s Napa and Sonoma counties report that the number of Pierce’s disease cases exploded in 2015. Photo: Jack Kelly Clark/University of California

North Coast, Calif.—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 has been developed in Texas and 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.


    Jan. 13: UC viticulture advisor Dr. Monica L. Cooper has planned an in-depth discussion of the PD problem. The talk will occur during a meeting of the Napa Valley Viticultural Technology group, but it is open to all concerned parties, on Jan. 13 from 10 a.m. to noon at the Century Theatres in Napa. (Registration is required.)

    Feb. 10 and 11: In Sonoma County, UC viticulture advisor Rhonda Smith will address PD at the UCCE 2016 Sonoma County Grape Day on Feb. 10 at the Wells Fargo Center for the Arts in Santa Rosa. She also will hold a meeting about PD the following day, Feb. 11, in the large conference room at the UCCE Sonoma County office.

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.

It’s not as much a factor north of Healdsburg, where the Alexander Valley is much wider and farmers grow other crops along the Russian River. “It’s also probably colder during the winter there,” McIlroy noted, adding, “and many of the streams that feed the river don’t have much riparian area.”

He said that Rodney Strong has removed a lot of invasive species like Himalayan blackberry and vinca major, which harbor the sharpshooters, but PD is often a problem around landscaped homes that feature plantings of non-native plants and irrigated lawns.

Also a menace in Napa County
Rhonda Smith’s counterpart in Napa County, Dr. Monica L. Cooper, also reports more PD. “There has been a marked increase in PD in 2015, with some growers reporting incidence rates as high as 50% of blocks, especially in regions where background PD pressure is historically high.”

In the North Coast, PD is generally associated with riparian areas or other landscaping that provides alternate hosts for the insect vectors.

Cooper said, “We have experienced a severe impact on vines, since systemic PD infections are generally lethal within a few years.” 

Steve Moulds, the owner of Moulds Family Vineyard in the Oak Knoll District of Napa Valley and current president of the Napa Valley Grapegrowers, agreed. “There has been a tremendous resurgence of PD this last year. We have discussed different factors, such as drought-induced stress. I do know that the Big Ranch Road area has been particularly hard hit, but even non-riparian areas near our ranch are feeling some effects.”

Cooper noted that scientists have some theories and potential explanations for the outbreak, but they are just gearing up to study them as part of the three-year grant recently awarded by the CDFA PD/GWSS Board to the team led by Dr. Rodriogo Almeida at UC Berkeley.

Cooper explained that PD epidemiology is complicated because researchers have to consider the vector, pathogen, vine, alternate host and weather/climatic conditions. “For example, temperature can influence transmission efficiency of the vector. Temperature and vine water status can also impact the pathogen’s colonization of the vine.”

She said that many factors could contribute to outbreak conditions. “One of the objectives of our research will be to study those factors to try and determine which are the most influential; that, in turn, should direct management practices.”

She continued that it typically takes five to 10 years for epidemiological studies like this to generate the volume of data necessary to make broad conclusions, but the researchers planned to do their best to address some of the pressing questions in the three-year time frame. Cooper said she hopes that they can continue to receive funding to study these issues.

Garrett Buckland is vice president of Premiere Viticultural Services and vice president of the Napa Valley Grapegrowers. “We deal with PD every year, but we’ve seen a bigger spike this year. It’s two to three times worse,” he said. “For us in Napa Valley, the odd patterns of weather are a bigger problem than the drought.”

Buckland added, “I’m not sure why PD was so bad, but this was the warmest winter on record, with temperatures in the 90°s in January. Perhaps this weird weather threw the sharpshooters off their usual life cycle.”

He added, “We can’t treat the disease at this point, so we have to go after the vectors, and that means getting rid of host plants and diseased vines or at least affected parts of them.”

Not an issue in Lake and Mendocino counties
North of Napa and Sonoma, Pierce’s disease doesn’t seem to be a problem. Glenn McGourty, UC viticulture & plant science advisor for Mendocino and Lake counties, said, “Mendocino and Lake counties have not had any issues to the best of my knowledge during this past season. We have found scattered PD in the past along the Russian River and some of its tributaries, but the disease doesn’t seem to persist locally.”

He added, “According to Dr. Sandy Purcell, retired entomologist from UC Berkeley, we have plenty of the vectors, including blue green sharpshooters and willow sharpshooters, but since temperatures routinely fall below 26° F, he thinks the bacterium doesn’t survive.”

McGourty noted, “Growers probably need to plant some of Andy Walker’s PD-resistant cultivars. Not much else is effective.”

A possible cure?
Growers are intrigued by recent research out of Texas A&M University and published in the June 2015 issue of the journal PLOS ONE. “Control of Pierce’s Disease by Phage” describes how a “cocktail” of phages, viruses that infect and replicate within a bacterium, can destroy the Xylella fastidiosa bacterium that causes Pierce’s disease. (Read Wines & Vines' coverage of the research in "Experimental Solution to Pierce's Disease.")

For more information
Viticultural advisor Cooper recommended the third edition of the UC ANR Grape Pest Management Guide as a great reference for all things Xylella, and it is currently available on sale through the UC ANR publications website.

As part of the funded research project, UC also is revamping the Xylella website, so that it will contain updated information, links to talks, etc. 


Posted on 12.08.2015 - 15:00:22 PST
In the halcyon days of Napa in the '70s and '80s, Pierce's disease was brought up as the killing rickettsia that could wipe out the valley grape industry. Not to be taken lightly by Drs. Amerine and Cook, et al. Let us get a handle on this, and not blame the weather.

Posted on 12.11.2015 - 14:42:22 PST
The issue IS the weather. That's the point of the article.

Posted on 03.02.2016 - 19:35:18 PST
PD is cyclical and predictive. The explosive outbreaks occur every 20 years in three-year peaks of infection and in the usual areas of previous historic outbreaks. Since 1970 in Napa Valley, PD ground-zero has been the riparian zones of Napa River, Chase Creek, Hopper Creek, Spring Mountain, among others. 1970-73, 1992-1995, 2015-?. Every 20 yrs. There are eradication protocols but most are not "environmentally PC." What worked best for me was "riparian habitat modification" - preapproved by the County & DFG of course. Rogueing non-native plant species that support the Xylella bacteria as well as the BGSS vector. Himalayan berry, French prune, Vinca... get rid of 'em. Replace with conifers, oaks, black walnut, native grasses. I've seen trap crop fences (St.George growing on a VSP trellis along a setback. Grower applies imidocloprid foliarly or via drip to kill BGSS vectors). Corn rows and shadecloth fencing is being utilized, don't know if effective. My advice: wipe out the bacteria source.
Rick Aldine