March 2015 Issue of Wines & Vines
 
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Wine Grape Grower's Assessment in 2015

Fifteen years of PD assessment with many possible solutions in sight

 
by PWV Staff
 
 
Andy Walker
 
Andy Walker looks over 10 new PD-resistant wine grape selections in November 2014.

Time may be running out for the California wine industry as we know it,” read the first line of the Wine Spectator cover story on Oct. 15, 2000. One look at the vineyards in Temecula, Calif., in 2000 proved it was a very believable statement.

practical winery vineyard
 

Between 1999 and 2000, 40% to 60% of the vineyards in Temecula were wiped out. The cause: an old foe of the wine industry, Pierce’s disease (PD), had a new vector, the glassy-winged sharpshooter (GWSS), which had hitched a ride to California from its native range that runs from Texas through Florida, and north to around the Mason-Dixon line.

GWSS was spreading PD in California as never before seen in modern times, first infesting Southern California and then moving north to Bakersfield and Tulare. Experts estimated that, unchecked, GWSS could very well infest regions as far north as southern Oregon.

What happened?
If the experts were right, then why is California still enjoying record wine grape harvests? In 15 years a lot has happened on many different fronts. Key has been the wine grape grower’s assessment, which wine grape growers first voted for in the spring of 2002 and overwhelmingly renewed again in 2005 and 2010. It is due for the next vote this spring.

When the assessment was passed, the PD/GWSS Board was created to direct how the funds were spent. The PD/GWSS Board has 15 members: eight growers, six grower/producers and one public member. An effort is made to ensure that the members represent all wine grape-growing regions of California.

Funds raised from the assessment are earmarked for research and other activities related to Pierce’s disease and glassy-winged sharpshooter. The assessment pertains only to grapes grown in California and crushed for wine, and it is a value-based assessment, which by law can be no greater than $3 per $1,000 of value. In 2001 the rate was set at $3 per $1,000. From 2002 to 2006 the rate was $2; $1.50 in 2007; $1 in 2008-09; 75 cents in 2010-11; $1 in 2012, and 75 cents in 2013-14.

“In the beginning of the PD/GWSS research program, basic research was needed because we did not know much about the disease or the vector,” explains Bob Wynn, statewide coordinator of the Pierce’s disease control program. “The results of that research have led us to focus on those areas of research that provide the best chance for development of a solution for the disease. The PD/GWSS Board is now targeting the most promising research areas for investment of grower assessment dollars, which has resulted in the need for less funding.”

The 2010 vote gave the PD/GWSS Board additional authority to spend assessment funds for research and outreach on other pests and diseases that threaten California’s wine grapes. Three pests and one disease have been designated since then: the European grapevine moth, vine mealybug, brown marmorated stink bug and grapevine red blotch associated virus (GRBaV).

Perhaps just as important is that the assessment illustrates the wine industry’s willingness to shoulder some of the burden. While in the past 15 years the wine grape assessment has raised more than $45 million, the local, state and federal governments have spent nearly $500 million on behalf of the industry to control and contain GWSS.

Citrus growers and the nursery industries in California also have played an important role in control and containment efforts. Treatments for GWSS in citrus fields have been critical since it is a major overwintering crop for GWSS. Pre-transport inspections of nursery stock to ensure they are GWSS-free prior to shipping is vital since those plants can provide a means for GWSS movement from infested areas to uninfected areas in the state.

Another critical part of the fight against PD and GWSS has been the PD control program managed by the California Department of Food & Agriculture (CDFA). The main goal has been to provide a continuous statewide survey to verify infested regions and to keep a watchful eye for new infestations.

Thirty infestations have been discovered and dealt with since 2000. The CDFA also works closely with the U.S. Department of Agriculture to manage the area-wide control program to keep GWSS numbers low in infested areas. Keeping those numbers down is key to preventing the spread of PD in those areas and helping prevent GWSS moving further north in California.

The CDFA has been key in developing a bio-control method to help contain GWSS in the form of tiny parasitic wasps that prey on GWSS eggs. More than 2.4 million wasps have been released at various locations around California, and at some sites the wasps have established themselves as permanent bio-control agents.

However, these programs were only intended to be temporary. Their purpose is meant to be a delaying action to buy time for science to find a permanent solution.

Research follows money
Prior to 2000, comparatively little research had been done on PD. While there were some PD hot spots around the state, most of California experienced little threat from PD, so only relatively small amounts of funding had been devoted to finding a solution.

That changed in 2001 with establishment of the wine grape assessment. Soon more than 100 research projects related to PD and GWSS were under way and bearing results. In the next five years more was learned about PD than in the previous 50 years. Much of this was basic research that laid a solid foundation for other research to be built upon.

Nearly 15 years later, positive results are coming out of field trials, and wine grape growers could have commercially available solutions within the next few years.

No cure is coming
Very early, researchers warned all the stakeholders involved not to expect a cure. A cure for infected grapevines would not be discovered for many reasons. However, there were several other methods that offered solutions to create grapevines that would be unaffected by the Xylella fastidiosa (Xf) bacterium that causes PD.

Traditional plant breeding and genetic modification were potential methods,but no one was sure what might work. The PD/GWSS Board elected to explore all viable options. To aid in this, the PD/GWSS Board established the PD/GWSS research scientific advisory panel (RSAP).

The RSAP is a group of scientists who review all the different research proposals and then make recommendations about those they believe are the most likely to meet with success. They represent scientists who work in different disciplines from across the United States. They do not have a stake in any PD/GWSS research and are as impartial as possible when making recommendations.

Possible solutions take root
After many years of work and many discoveries about grapevine genetics and the mechanisms of how some other plants cope with the PD-causing Xf bacterium and survive, scientists began testing their theories—first in labs, then in greenhouses and finally in real-world vineyards around the country. A few that seemed to work well in the lab or greenhouse failed when tested in field trials, but several have proved to be successful. Two are currently headed toward commercialization.

One of the first solutions likely to be available to growers will be PD-resistant wine grape vines produced by traditional breeding methods. The work has been spearheaded by Dr. Andy Walker at the University of California (UC), Davis. The genes that made native North American grapevines (Vitis arizonica) resistant to PD were identified. Those vines were crossed with European wine grape varieties (Vitis vinifera) to pass that gene along. Using high-tech equipment, when the new plants are little more than seedlings in the greenhouse, a leaf is tested for the presence of the gene that provides the resistance to PD. Those that have the gene are advanced to the next level of breeding.

Thanks to the use of high-tech equipment, traditional plant breeding that once took years can now be accomplished in only 18 to 24 months. Trial plantings to test fruit quality and for winemaking have proven that these grapes have retained flavor profiles to be expected from non-Xf-resistant wine grapes grown in California.

Ten scions, along with three PD-resistant rootstocks, were released to the Foundation Plant Services at UC Davis, the first step in making them available to wine grape growers. Of note is that while these vines are PD-resistant, and the grapes look and taste like Chardonnay, Sauvignon Blanc or Zinfandel, genetically they are not the same, so they cannot be called by the same names. However, they can be planted in vineyards or areas of vineyard that have a history of PD and then used as blending grapes to maintain a desired flavor profile.

What if you could give a shot to a grapevine to give it the ability to prevent PD? Dr. Donald L. Hopkins of the University of Florida discovered a benign strain of the Xf bacterium on the East Coast and has used it as a biological control of PD. Vines planted in the southeastern United States were inoculated with this benign strain more than 10 years ago and have shown no symptoms of PD.

Vines in California were infected with the benign strain at three sites in 2008. Promising results were obtained in a Riverside test in 2013, but it is too early for conclusions to be made from that trial. Further work is now being conducted by a private company for possible commercialization of the benign strain.

Four other methods that all involve genetic modification of the rootstock and not the scion have shown varying degrees of success in field trials. In each trial the roots were modified to create a protein or molecule that is passed across the graft and up into the scion to disrupt Xf and allow the vine to continue to thrive with little or no signs of PD.

• Dr. Stephen Lindow at UC Berkeley discovered that Xf in plants is influenced by a small diffusible signal factor (DSF) protein that accumulates when cells are at high cell density. By enabling the vine to produce its own DSF, Xf bacterial reproduction is disrupted; its ability to efficiently colonize the grapevine’s water-conducting tissue is interrupted, and PD symptoms are reduced or prevented. Lindow has also begun work on a DSF strain that could be used on existing vineyards to prevent PD.

• Dr. David Gilchrist at UC Davis is controlling programmed cell death (PCD) in grapevines from Xf. Research identified the genetic instructions that, if altered, resulted in suppressing PD symptoms in grapevines. Field trials have led to identification of two gene sequences that significantly reduce Xf in infected grapevines.

• Plant cell wall-degrading enzymes including polygalacturonase (PG) allow Xf to break cell walls and spread within the grapevine. Researchers discovered that some plants express their own PG-inhibiting proteins (PGIPs) to protect themselves and limit the bacterium’s ability to spread. Dr. John Labavitch and his UC Davis team discovered several such gene candidates, and they are now working to identify which will be the most effective in halting Xf in grapevines. This research might also lead to a spray or drip system application for established vineyards.

• Dr. Abhaya Dandekar at UC Davis found that a key feature of Xf is its ability to digest pectin, permitting it to move within the grapevine. Proteins like polygalacturonase-inhibiting protein (PGIP) and a chimeric antimicrobial protein have been proven to prevent or reduce the movement of Xf in grapevines. Two field trials showed that resistance created in the rootstock moves up into the scion.

These genetically modified rootstocks have shown varying degrees of success and are continuing to be developed in field trials.

Solutions are near
It is hard to say exactly what form the solution to PD will take. With several of them in development, it may be a combination, or perhaps one or another may become preferred by region, variety or growing practices.

During the 2013 Pierce’s disease Research Symposium in Sacramento, Calif., Dr. Gilchrist said, “We are halfway down the field toward a touchdown, and the next step is critical. With a little bit of funding and hard work, we can turn our work into something practical.”

It is clear the growers’ money has been well spent, and there is good reason. Al Rossini, a grapegrower from Denair, Calif., and a member of the PD/GWSS Board, says, “When the guys putting up the money are the same ones who are making the decisions, you get results.”

 
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