I t was 2000 and the combined threat of Pierce’s disease (PD) and the glassy-winged sharpshooter
(GWSS) was just becoming evident with the massive loss of vineyards in Temecula. Winegrape growers moved swiftly and voted to assess themselves to raise money for research targeted at finding a solution to the threat that many said could devastate California’s wine industry.
So why hasn’t GWSS spread throughout the state as many thought they would? Part of the answer is because of the
USDA’s GWSS Area-Wide Control programs, that have been limiting the spread northward from GWSS-infested areas in the south. in Kern County. And since GWSS often overwinter in citrus groves, the cooperation of citrus growers has also been key in limiting the spread of GWSS in these area.
“What the Area-Wide Control programs do is give us time,” said PD/GWSS Board Chair Steve Schafer. “It is the time we have needed to fund and conduct the vital research necessary to find a solution to Pierce’s disease.”
The Area-Wide Control programs take different approaches to controlling GWSS depending on the area of the state. Working with local county ag departments and the PDCP, the goal of the programs is to suppress GWSS populations in order to reduce the incidence of Pierce’s disease and the movement of GWSS, the key vector in the spread of PD. In GWSS-infested areas of Kern and Tulare counties, the goal is to reduce the natural spread of GWSS northward into un-infested areas. The Area-
Wide Control programs also target the commercial citrus in these counties
Vineyards in these counties benefit because keeping population levels of GWSS low in infested areas means there is less chance of natural and/or artificial spread of the pest and PD to areas either un-infested or areas under active control. This fact was proven in research conducted by a UC Extension viticulturist
Citrus is a prime overwintering home for GWSS, and in the many places in Southern
California citrus groves and vineyards are found side-by-side. Thus GWSS treatments in citrus are a very important part of the Area-wide Control programs.
Now, nearly 15 years later, possible solutions are undergoing field trials and PD-resistant winegrapes have been developed and continue on the road to validation and commercial availability.
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S P R I N G 2 0 1 4 b u l l e t i n
W hen the PD/GWSS Board began funding research over a decade ago, they wanted to invest in as many promising approaches as possible, not knowing where the solution to Pierce’s disease (PD) would lie. One avenue researchers have pursued is modifying grapevines in the laboratory to be resistant to Xylella fastidiosa (Xf).
One of those researchers is David Gilchrist from the
Department of Plant Pathology at UC Davis. His research has centered on preventing the death of cells caused by Xf. Research has shown that many plants can coexist with Xf and not show any harmful signs, so work began on several fronts to find ways to transfer this ability to winegrapes.
Cell death-related PD symptoms occur when a plant’s genes controlling cell death are activated in the presence of Xf. In a large-scale test of thousands of genes, Gilchrist’s group identified several genes that suppress PD symptoms and reduce bacteria levels, and introduced them into otherwise vulnerable grapevines in the laboratory.
After first testing the PD resistance of the modified grapevines in a controlled greenhouse, Glichrist’s approach and that of other researchers was to test over
Grapevines in the field trial being mechanically inoculated with Xylella fastidiosa.
300 modified grapevines for Xf resistance under field conditions in two locations. The Riverside County field experiment was planted in spring 2011,
and plants were exposed to infection via natural exposure to glassy-winged sharpshooters. The field trials in Solano County began in summer 2011 with both modified and non-modified grapevines mechanically infected with Xf.
“The results after two years were quite positive.
A number of the genes were in fact providing
protection compared to the control plants, which showed symptoms of PD and died,” said Gilchrist.
Testing over the past two years confirms that all of the grapevines at both sites are infected with Xf.
The non-modified grapevines show symptoms of
PD and cane or plant death, while each of the modified grapevines show suppression of symptoms and reduced bacterial counts. The researchers hope to continue collecting data at both sites over the next two years to determine the sustainability of PD-symptom suppression.
Continued on page 2
I N S I D E T H I S I S S U E
P A G E 2
• Legislation to re-authorize the
Pierce’s Disease Control Program
(PDCP), the PD/GWSS Board,
PD/GWSS winegrape assessment and the PD Advisory Task Force has passed its first committee vote.
• The PD/GWSS Board has retained Mark Greenspan to serve as a researcher and grower liaison.
P A G E 3
• Researchers identify genetic markers that are tightly linked to
Pierce’s disease (PD) resistance and have allowed classical breeding to be greatly expedited.
• Research tests two alternative
GWSS control strategies to complement those currently in place or being developed.
• A look at the impact a mating disruption or sterilization program of GWSS might have on its natural enemies.
P A G E 4
• Find out how the USDA’s GWSS
Area-Wide Control programs, have been limiting the spread northward from GWSS-infested areas in the south.

- continued from page 1
In addition, PD-susceptible grapevine scions have been grafted onto rootstocks modified with Xf-resistant genes to determine if the Xf-resistant genes from the rootstocks are able to render the entire plant resistant to PD without modifying the fruit-bearing portion.
“If the XF-resistant rootstocks protect the entire vine, then growers could potentially graft any of their desired cultivars or varieties of grapes onto the rootstocks and have
PD-resistant vines in their vineyards,” said Gilchrist.
Left - David Gilchrist transfers young vines to a field trial test vineyard in 2011.
Right - Researchers survey vines in one of the field trial vineyards.
A ssemblyman Wesley Chesbro (D-Arcata) has introduced AB 1642, legislation to re-authorize the
Pierce’s Disease Control Program (PDCP), the PD/GWSS Board, the PD/GWSS winegrape assessment, and the PD Advisory Task Force. The bill is being sponsored by the California Association of Winegrape Growers, the Family Winemakers of California, and the Wine Institute.
If the bill is passed this year, a referendum would be held in 2015 (EGVM) was funded when it threatened California’s winegrape allowing California‘s winegrape growers to decide if they want the
PD/GWSS assessment and the PD/GWSS Board to continue for another five years. The assessment is used to fund research and other activities related to Pierce’s disease and the glassy-winged sharpshooter, which spreads PD, and may, when certain conditions are met, be used for research and outreach on other pests and diseases that threaten California’s winegrapes. Under this provision, research and outreach on the European grapevine moth industry.
In March, AB 1642 passed its first committee vote with a unanimous 6-0 vote from the Assembly Agriculture Committee. During the testimony on AB 1642, the bill’s co-sponsors thanked Assembly member Chesbro for his continuing efforts and leadership on this issue. The bill has picked up 17 co-sponsors in the California Assembly and the Senate, and now heads to the Assembly Appropriations Committee for a review and vote.
M ark Greenspan, Ph.D., has been retained by the PD/GWSS
Board to serve as a researcher and grower liaison. He will provide a wide range of advice and guidance on research efforts and field trials.
“My goal is to help ensure that current and future research activities yield maximum benefit to California’s winegrape growers,” said Greenspan.
Greenspan holds a master’s degree in horticulture/viticulture and a doctorate in agricultural engineering, both from UC Davis. He is also a Certified Crop Advisor and Certified Professional Agronomist, with nearly 25 years of viticultural experience, comprised of scientific, technical, and practical field activities.
Greenspan has written a number of scientific and trade journal articles on grapevine irrigation practices and has extensive experience in vineyard mineral nutrition, crop load management, vineyard uniformity, grape maturation, weather, climate, and viticultural technologies.
P A G E 2
Mark Greenspan
Principal Investigator: Andrew Walker, Dept. of Viticulture and Enology, UC Davis
Genetic maps were used to identify genetic markers that are tightly linked with Pierce’s disease (PD) resistance and have allowed classical breeding to be greatly expedited. This has enabled construction of physical maps, which are used to identify resistance genes and thus enable researchers to identify candidate genes responsible for PD resistance. Researchers also developed plant tissue cultures of PD-susceptible Chardonnay and Thompson Seedless, and the rootstock St. George for modification to verify candidate PD-resistance gene function.
Modified tobacco plants have been tested against Xylella fastidiosa in the greenhouse and promising results have been obtained with two candidate genes. Modified grape plants have been acclimated to greenhouse conditions and are ready for testing. Although the current modified grape plants were produced using the traditional procedure, the researchers are also testing another technique to speed the development of modified tissue that will allow gene candidates to be tested faster.
Chardonnay embryos growing in germination medium.
In vitro inhibition assay used to evaluate fungi. A)
Xf-only control; B) Fungus showing no Xf inhibition;
C) Fungus showing moderate Xf inhibition; D) Fungus showing total Xf inhibition after 8 days of incubation.
Co-Principal Investigators: Philippe Rolshausen & Caroline Roper, UC Riverside
Currently, successful management of Pierce’s disease (PD) largely involves vector control through the use of insecticides. This research tests two alternative control strategies to complement those currently in place or being developed. The first strategy is to use biocontrol agents that can be introduced in grapevine cuttings prior to field planting. The researchers have identified eight fungi and one bacterium naturally inhabiting grapevines that are hostile to Xylella fastidiosa (Xf). They are currently evaluating in a greenhouse assay the ability of these biocontrols to reduce PD-symptom progression as well as the concentration of Xf in vines. The second strategy is to use the natural compounds produced by the eight fungi as a curative treatment for PD. The researchers are currently extracting and characterizing these fungal compounds. Thus far, they have identified three compounds that are inhibitory to the bacterium in an in vitro assay. These natural compounds and fungi are currently under review for patentability.
Principal Investigator: Rodrigo Krugner, USDA ARS, Parlier, CA
Unmated glassy-winged sharpshooter (GWSS) females deposit unfertilized eggs, and mated females need to re-mate to continue to produce progeny. Therefore, it is possible that egg parasitoids of
GWSS, used in California to control GWSS, are utilizing unfertilized eggs for reproduction in mass rearing facilities and/or in the wild. Research looked at what effect this might have on parasitoid control of GWSS. It was found that parasitoids failed to emerge as adults from unfertilized eggs more often than from fertilized eggs. Results suggest that reducing rates of GWSS egg fertilization through a mating disruption or sterilization program will have a negative impact on control of GWSS by natural enemies.
Wasp parasitoids lay their eggs into a
GWSS egg mass and have been effective in aiding in the control of GWSS.
P A G E 3