Within the next decade, the results of traditional and laboratory (GMO) research could begin to remove one of the most ferocious disease threats to California’s grape industry - Pierce’s disease - vectored by the glassy-winged sharpshooter (GWSS) pest.

“We are almost there in bringing to market the first round of solutions for the Pierce’s disease problem,” said Robert Wynn, statewide coordinator for the Pierce’s Disease Control Program, administered by the California Department of Food and Agriculture (CDFA).

“We have the technologies now available to control Pierce’s disease and want to commercialize the science for growers,” Wynn told wine grape growers at the Allied Grape Growers annual meeting in Fresno, Calif.

Within 3-5 years, new cultivars developed through traditional breeding could be on the commercial market. Disease resistant cultivars developed using biotechnology genetically mordified methods (GMOs) could be available to growers about a decade after that.

Wynn said, “Research dollars have produced a robust pipeline of diverse technologies including a strong portfolio of disease management strategies using different modes of action which is important over the long run.”

Pierce’s disease is caused by the bacterium Xylella fastidiosa which plugs the water-conducting system (xylem) in plants, according to the UC Davis Pest Management Guidelines website.

Bacteria are spread from plant-to-plant by xylem-feeding insects, primarily the GWSS.

Symptoms appear when a significant amount of bacteria build up in the plant. By mid-season, some or all fruit clusters on an infected cane can wilt and dry. The tips of canes and roots may die back.

Up to now, insecticides have been the primary weapon to thwart bacteria transmission by the sharpshooter.

Joint approach

Launched in 1999-2000, the CDFA-led Pierce's Disease Control Program involves multi-agency cooperation, along with academic institutions and industry organizations. It was established to prevent Pierce’s disease and the GWSS from further damaging California vineyards beyond the damage done in the Temecula area of Riverside County at the time.

The program is a local and national program involving USDA, CDSA, county Ag Commissioners, the University of California, industry groups, stakeholders, and the public.

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Significant funding for the program comes from the federal government.

In California, the research program is funded largely by mandatory grower assessments based on crop value. The 2014 assessment rate is 75 cents per $1,000 crop value (the max is $3 per $1,000 value). Only California grape growers pay the assessment.

Almost $46 million has been collected from California growers over the lifespan of the program. About half - $24 million – has funded basic research on Pierce’s disease and its GWSS vector, plus another $5 million spent on applied research.

The 15-peer-member Pierce’s disease and Glassy-winged Sharpshooter Board, consisting of eight wine grape growers, six producer-processors, and one public member, advises CDFA on how the assessment funds should be spent.

Solutions ASAP

Wynn’s goal is to provide solutions to growers as soon as possible. He views Pierce’s disease solutions as “insurance against future devastating outbreaks and protecting California’s and the nation’s wine grape industries.”

Why pursue more than one potential solution?

“Vines are kind of funny,” Wynn explained. “One technology might work for a while and then stop working. We have developed a robust portfolio for a long-term solution.”

UC Davis Viticultural Geneticist Andy Walker developed the traditional breeding approaches which so far has produced 15 cultivars on three root stocks which are on the path to commercialization at the UC Davis Foundation Plant Services. The cultivars are resistant to Pierce’s disease.

“Of the cultivars, seven are 94 percent Vitas vinifera, and eight are 97 percent Vitas vinifera,” Wynn said.

This makes them very close to the types of grapes used to produce excellent California wines.

Walker also has an ongoing pipeline to develop more resistant vines and improved crosses for future use.

Transgenic approach

Transgenic research to help thwart Pierce’s disease includes four GMO-based strategies. These approaches are under development by several researchers at the University of California.

1 – ‘Gum ‘em up’ method’ – Two different strategies are both designed to prevent the spread of Xylella fastidiosa bacteria throughout the vine. In one strategy, an adhesive protein gene would prevent the bacteria’s ability to spread in the plant. A second approach tricks the bacteria to remain aggregated and unable to move. By preventing the bacteria from spreading throughout the vine, these strategies are expected to keep the plant alive.

2 – ‘Search and destroy’ method - Proteins from grape and fruit sources are being identified with the optimal combined capability to locate and disable Xylella bacterias.

3 – ‘Grape life support’ – Native grape genes and genetic material are identified to prevent vinifera grape vines from dying when infected with Xylella. These sequences appear to prevent a ‘programmed cell death’ reaction that vinifera grapes are more susceptible to undergo than other grape varieties facing Xylella infection.

4 – ‘Passport denied’ – This approach prevents Xylella movement across xylem cells by shutting down a bacterial enzyme that would otherwise enable the Xylella bacteria to move from one xylem cell to another and hence throughout the vine.

Wynn says the grapevines developed with traditional breeding methods could be available to growers within the next several years. Plants developed using laboratory methods will take longer.

On-going solutions

He says the search for solutions must be ongoing and not end by a certain date since growers deal with pests every day.

“Pest and disease control programs are continuous and ongoing, and involve a certain degree of uncertainty,” Wynn said.

“The strategy is to continue to search for solutions so we can provide growers with a choice of methods offering robust protection for decades to come.”

The disease and vector first became an issue in California in the late 1990’s when large GWSS populations killed grape vines in southern California by spreading Pierce’s disease.

Since then, the CDFA program has found and eradicated small infestations in Solano, Contra Costa, Sacramento, San Joaquin, Santa Clara, and San Luis Obispo counties. To date, 17 infestations have been eradicated with insecticides and other methods.

Biological control is also a critical method for managing GWSS. Over the life of the program, CDFA has released more than 2.4 million parasitic wasps at 146 release sites.

Presently, GWSS infestations are found mostly in southern California (except Imperial County), plus the Central Valley’s Kern and Tulare counties and portions of Fresno and Madera counties.

The sharpshooter travels by natural movement (flying) and by other means - hitchhiking on transported nursery stock and bulk citrus.

Citrus-Huanglongbing link?

Since Pierce’s disease is a bacterial disease spread by insects, researchers believe much of the research conducted on it may also benefit the search for solutions to the citrus bacterial disease Huanglongbing and its primary vector, the Asian citrus psyllid.

A referendum on the continuation of the PD-GWSS Board and assessment is scheduled for next year. To continue, at least 65 percent of the growers voting must vote yes and must have paid the majority of the total assessment of those who voted; or a majority of the vote must be yes with 65 percent of the total assessment paid by those who voted yes.

For more information, contact Wynn at [email protected] or (916) 654-0433.