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2014 PD Research Symposium Highlights

PD-Resistant Cultivars Near Release, Foliar Sprays for PD Control Under Development
by Ted Rieger
December 23, 2014
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Dr. Steven Lindow of UC Berkeley presented information at the 2014 PD Research Symposium about the potential for using "pathogen confusion" for disease control by restricting movement of the PD bacterium in grapevines.

The 2014 Pierce’s Disease (PD) Research Symposium held in Sacramento December 16-17 highlighted research progress and promising new products, technologies and mechanisms to prevent and control PD in grapevines. These include: traditionally-bred PD-resistant cultivars, the development of microbiological and transgenic mechanisms to suppress PD, and new directions to control PD’s vector--the glassy-winged sharpshooter (GWSS).

The 2014 symposium was the 13th research symposium held since the California Department of Food and Agriculture (CDFA) PD and GWSS Board was formed in 2001 to collect and allocate winegrape industry assessment funds. Since that time, the Board has spent more than $30 million for research projects and related activities.

PD Suppression Using Pathogen Confusion
Dr. Steven Lindow of the UC Berkeley Department of Plant and Microbial Biology is investigating several methods to utilize pathogen confusion to control and suppress PD in grapevines. Lindow’s research shows that certain fatty acid molecules can inhibit the movement of the PD causing bacterium, Xylella fastidiosa (Xf,) within a grapevine to confer PD resistance.

Xf produces an unsaturated fatty acid signal molecule called diffusible signal factor (DSF). Accumulation of DSF in Xf cells causes a change in many genes in the pathogen, but the overall effect is to suppress its virulence in plants by increasing its adhesiveness to plant surfaces and by suppressing the production of enzymes and genes needed for its active movement through the plant. Lindow summarized, “Our objective is to artificially increase this molecule within the plant because it suppresses the ability of Xf to move within the plant.”

Several DSF unsaturated fatty acid molecules have been identified and investigated, including palmitoleic acid, that is affordable and commercially-available. Lindow is currently testing the efficacy of foliar spray applications of palmitoleic acid combined with surfactants to penetrate leaf and petiole surfaces on PD-infected vines in greenhouse experiments.

Lindow is also experimenting with other products and methods utilizing DSF to suppress PD in grapevines. His lab has produced transgenically-modified grapevine cultivars with an Xf gene that increases production of DSF molecules, and these cultivars are being evaluated in greenhouse and vineyard field trials. Lindow is also evaluating the potential for biological control using an endophytic bacteria that would grow within a grapevine to produce DSF molecules. This bacteria could potentially be introduced through a spray application.

Lindow observed, “We are excited about results to date that show that several means of elevating DSF levels in plants have provided disease control via a strategy of ‘pathogen confusion.’” He added, “Control of Pierce’s Disease by direct application of DSF is a very attractive disease control strategy since it could be quickly implemented and would utilize commonly used agricultural equipment and methods and would not require the use of transgenic technologies.”

Traditionally-Bred PD-Resistant Cultivars Advance
University of California, Davis (UCD) Department of Viticulture and Enology Professor Dr. Andy Walker is traditionally-breeding winegrape varieties of Vitis vinifera with the native American Vitis species that carry PD-resistance genes. The resulting vines are the most promising PD-resistant cultivars with Vitis vinifera parentage and quality produced to date, and will be the first PD-resistant cultivars commercially available from the CDFA PD research program. Walker announced a red cultivar with 94% vinifera parentage will be the first commercial release, expected sometime in 2015. This cultivar includes vinifera parentage of Petite Sirah and Cabernet Sauvignon and has been a consistent favorite among red wines produced and tasted from field trials of resistant cultivars.

Walker uses a traditional breeding method of backcrossing through several generations of crosses to progressively increase the percentage of vinifera parentage and characteristics to 97% with the final cross. Each generation carries resistance genes.

Wines have been produced in recent years from field trials of these cultivars and evaluated by UCD staff and wine industry representatives at several tastings. Some cultivars at the 88% vinifera level begin to show acceptable quality, and at the 94% and 97% levels they show vinifera character and some have been considered commercial quality.

Walker began advancing PD-resistant cultivars to UCD Foundation Plant Services (FPS) in 2013. To date, 13 scion cultivars are being prepared for possible release that include 94% and 97% vinifera parentage of varieties that include Chardonnay, Cabernet Sauvignon, Zinfandel and Petite Sirah, along with three PD-resistant rootstocks. Not all selections advanced to FPS (currently or in the future) will be released for commercial use.

Walker’s lab continues to produce and evaluate multiple selections for PD resistance and for viticultural and wine quality characteristics. Walker is also involved with similarly breeding cultivars with genes for powdery mildew resistance. His future goal is to breed cultivars that have resistance to both PD and powdery mildew.
In addition to UCD campus field trials, a trial with PD-resistant cultivars has been in place in Yountville at a site with PD pressure in a riparian area that harbors the blue-green sharpshooter, a PD vector in the North Coast. Two new field trials were planted this year in Napa Valley. A new field trial was planted in Temecula in May 2014, with eight PD-resistant cultivars—seven red and one white--grafted onto three different PD-resistant rootstocks, and the commonly used 1103P rootstock. Out-of-state field trials have been ongoing in Texas and Alabama with earlier PD-resistant cultivars of 88% and 94% vinifera parentage since 2011, and additional field trials were planted in 2014 with newer generation cultivars.

Walker believes PD-resistant cultivars could achieve the most commercial use and acceptance, and help expand winegrape production, in the southeast US where growers are limited to growing currently available PD-resistant hybrids that do not have vinifera wine quality. He noted that Texas winegrowers are already requesting releases of 88% vinifera cultivars successfully tested in field trials that also carry the added benefit of downy mildew resistance.

An issue in producing varietal wine from a PD-resistant cultivar is what to label it and how to market it to consumers, as it will be unable to bear a common vinifera name (such as Chardonnay, Zinfandel, etc.). In California, Walker believes these cultivars would be planted at sites where PD pressure is high, and used at levels of 25% or less in varietal wines, or as a component in non-varietal red or white blends. However, Walker said, “I don’t think the name will make a big difference in the southeast, as many growers are willing to rapidly adopt these cultivars due to their high quality compared with the cultivars they currently produce.”

New Directions in GWSS Vector Management
To date, GWSS control has been primarily managed with applications of chemical insecticides, and to a lesser degree, with biocontrol using releases of parasitic wasps. Researchers are exploring other methods of GWSS control for two reasons. One is a general concern about GWSS developing resistance to currently used chemicals. The second reason is future availability of neonicotinoid classes of chemicals, such as the highly-effective and commonly used imidacloprid, targeted for possible regulatory restrictions due to potential impacts on bee populations.

Rodrigo Krugner of the U.S. Department of Agriculture Agricultural Research Service based in Parlier, Fresno County is investigating GWSS communication mechanisms and signals for the potential of developing mating disruption methods and technologies. Krugner said GWSS communicate exclusively by vibrational signals generated by tymbal organs in their legs that create low frequency waves that are undetectable by humans. In a laboratory setting, Krugner is using video surveillance and recording and analyzing male and female GWSS produced communication signals that the insects use to locate each other in order to initiate mating. “We want to determine what signals may be useful to disrupt the conversation between males and females,” Krugner said.

Krugner plans to experiment with playing disrupting signals in a commercial vineyard as early as 2015, and hopes to eventually develop field devices for commercial use. Krugner said, “We don’t know yet whether a field device would be used to repel, or attract and trap GWSS, or simply disrupt mating, but the technology is available to produce and amplify these communication signals for GWSS in vineyards.”

Dr. George Kamita of the UCD Department of Entomology and Nematology is investigating the potential of using the process of ribonucleic acid interference (RNAi), a gene regulatory process that in insects is used to maintain and regulate defenses against viruses. RNAi can be artificially manipulated to knock down the activity of a targeted gene. For this project, an RNAi-based method will be developed to knock down the activity of genes in the GWSS endocrine system that regulate juvenile hormone levels and will lead to aberrant GWSS development.

Kamita believes this could be applied in a sprayable form in conjunction with an insect virus that already inhabits GWSS under natural conditions and would serve as the mechanism to introduce the RNAi into the insect in its early nymphal stages. The end result would be to prevent GWSS nymphs from reaching maturity and reproducing. Kamita said two chemical companies are currently working to develop sprayable RNAi insecticides for control of chewing insects, and this technology could conceivably be adapted for sucking insects such as GWSS.

The 2014 PD Research Symposium was attended by nearly 100 PD researchers and plant pathology professionals, most of them from California, but also representatives from New York, Italy, and Brazil. The full, 258-page CDFA document of the 2014 PD Research Symposium Proceedings is accessible online at: http://www.cdfa.ca.gov/pdcp/Documents/Proceedings/2014_Proc.pdf

 

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