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PD Resistant Vines Available in 2015?

Pierce's Disease Research Symposium highlights progress and new tools

by Jon Tourney
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UC Davis researchers (from the left) Andy Walker and David Gilchrist, and biotechnology business consultant Peter Matlock presented information on research, development and commercialization of Pierce's Disease-resistant grapevines at the 2013 Pierce's Disease Research Symposium in Sacramento.

Sacramento, Calif.—The first commercially available Pierce’s Disease (PD)-resistant winegrape cultivars could be released by the University of California, Davis, Foundation Plant Services (FPS) for propagation by commercial nurseries as early as 2015.

These PD-resistant grapevines are the results of research funded through the California Department of Food and Agriculture’s (CDFA) Pierce’s Disease/Glassy-winged Sharpshooter (GWSS) Board, established in 2001 and through California wine industry assessments. The anticipated 2015 release was announced this week at the Pierce’s Disease Research Symposium held in Sacramento by CDFA, an annual forum attended by researchers from throughout the U.S. to discuss progress and exchange information.

Traditional breeding yields results
After 10 years of research, breeding and field testing of PD-resistant grapevines, UC Davis viticulture professor and grape breeder Dr. Andy Walker turned over the first 15 PD-resistant cultivars and rootstocks to FPS in 2013 to begin the testing and certification process for eventual release to commercial nurseries and grapegrowers.

Walker has been collecting and testing native American Vitis specimens for PD-resistance with a focus on Vitis arizonica, a grape species native to the Southwest U.S. and Mexico. Walker’s lab identified a gene with strong PD resistance (PdR1) from Vitis arizonica/candicans, a sub-species found in Mexico. Winegrape varieties of Vitis vinifera are traditionally-bred with V. arizonica over five generations of crosses to progressively increase the percentage of vinifera genes and characteristics. The resulting vinifera percentage is 50% with the first cross, and increases to 97% vinifera with the fifth and last cross, with each generation carrying the resistance gene.

Small scale lots of wine from the resistance-bred cultivars have been produced since 2010, beginning with 87% vinifera crosses, and Walker has held small tastings for wine industry groups. "The 87% vinifera wines received more negative comments, still retaining herbaceous characters of the American Vitis," he observed. "Then the 94% lost the negative characters and had fewer negative comments, and now with the 97% we've had complimentary comments, with higher quality vinifera character wine."

The most recent wines were generally well-received by members of the wine industry at tastings this year throughout California in Sacramento, Santa Rosa, Napa Valley, Temecula and Healdsburg.

The cultivars now at FPS include three rootstocks with the resistance gene; multiple selections of Chardonnay, Zinfandel and Petite Sirah; a selection of Cabernet Sauvignon; and crosses of Cabernet Sauvignon x Carignane. These cultivars include both 94% vinifera, and 97% vinifera. Walker will continue to advance more material to FPS that shows good promise, including newer material with more recently identified resistance genes, and possibly material with multiple resistance genes in the future.

In California’s North Coast, where the blue-green sharpshooter vectors PD into vines adjacent to riparian areas, PD-resistant vines could be planted as buffers in susceptible locations to produce grapes for blending into wines produced from 100% vinifera. The cultivars could also be effective in Southern California vineyards were GWSS populations are established, and in vineyards adjacent to citrus orchards that harbor GWSS populations. These cultivars could also be of great interest in other wine producing states in the south and southeast U.S. where high PD pressure prevents the successful growing of vinifera varieties.

Commercializing PD-resistant technologies
Peter Matlock, a technology and business development consultant based in Davis, Calif., entered a contract with the PD/GWSS Board in July 2013 to assist with commercializing technologies resulting from board-funded research. Matlock has been a president and founder of biotechnology companies and has experience commercializing research products.

Matlock described the research products and technologies funded by the PD/GWSS Board as a portfolio offering growers a range of choices, with both shorter-term and longer-term solutions. He said Walker’s conventionally-bred cultivars would be the first to market, with the fewest regulatory obstacles, and would be the most easily accepted. He described the transgenic research technologies being developed (to produce genetically modified grapevines) as backup strategies for long-term protection.

The process for development of both conventional and transgenic vine materials involves multiple steps: lab research, greenhouse research and trials, early field trials, advanced field trials, winemaking trials, and certification by FPS.

Matlock and Walker hope to begin larger-scale field trials through UC Cooperative Extension at three sites in commercial vineyards in California with up to six of Walker’s cultivars. The tentative plan is to plant rootstocks in 2014, and graft PD-resistant cultivars onto the rootstocks in spring 2015. "A goal of this project is to make growers more aware of the availability of PD-resistant cultivars," Matlock said. Larger plantings could also produce more wine for sampling to increase awareness of its quality characteristics.

Transgenic strategies and field trials
Matlock described four promising transgenic strategies in layman-friendly general terms based on how they function in the grapevine to combat and resist the PD bacterium Xylella fastidiosa (Xf) and to suppress PD sympto ms:

• "Gum 'em Up" increases the adhesiveness of Xf to plant surfaces to suppress its movement and virulence in grapevines;

• "Search and Destroy" uses grapevine engineering to produce an anti-microbial protein that kills Xf;

• "Grapevine Life Support" uses newly identified grape genes that can disrupt programmed cell death in order to suppress PD symptoms;

• "Passport Denied" uses proteins to inhibit the movement of the Xf bacteria in the grapevine.

The transgenic vine materials have significantly more regulatory hurdles to overcome than the Walker-bred materials, both in field trial permit requirements, and in commercialization, plus potential issues with grower and consumer acceptance. "I would not expect any of these transgenic technologies to be commercially available in less than 10 to 11 years," Matlock said. 

Early field trials are underway in Riverside County where GWSS populations and PD-pressure exist, and in Solano County near Davis, where researchers inject Xf into control and transgenic vines for evaluation.

UCD plant pathologist and researcher Dr. David Gilchrist noted that current trials with transgenic vines are being conducted with Freedom, a rootstock cultivar, and Thompson seedless (TS), most commonly a table grape cultivar. Gilchrist said these cultivars are used for early trials because they are easier to genetically transform, and since TS produces no seeds that could potentially spread, it has fewer requirements for permitting and management as a GMO field trial. Once the transgenic technologies prove successful with these cultivars, they can be transformed into and tested with winegrape cultivars that are more commercially desirable.

The next likely cultivars to evaluate are rootstocks 1103P and 101-14, and scion varieties Chardonnay and Cabernet Sauvignon. Researchers are also evaluating the potential to "stack" resistance genes, or combine transgenic resistance traits within the same cultivar to offer multiple layers of resistance and even greater PD protection. The 2013 PD Research Symposium Proceedings are accessible through the CDFA PD Control Program website at


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