Could This Biological Control Help Save the Wine Industry?
Whether you often end a long, stressful week with a savory glass of ‘Pinot Noir’ or produce the grapes that make it all possible, you understand the importance of a productive and prolific wine industry. Unfortunately, many growers have experienced severe losses in recent years. The culprit? Pierce’s Disease (PD), a particularly destructive grapevine ailment that reduces and limits grape production.
“It is an endemic, chronic problem in the southeastern U.S., where it is the primary factor limiting the development of a grape industry based on the high-quality European-type grapes that are grown in California,” says Dr. Don Hopkins, a plant pathologist with the University of Florida’s Institute of Food and Agricultural Sciences.
Hopkins, who has helped develop a biological control to fight the problem, says Pierce’s Disease has also limited the production of European grapes in Mexico, Central America, and the Southwestern U.S. California vineyards are at a greater risk due to the introduction of a pest — the glassy-winged sharpshooter — that transmits the bacterial pathogen, Xylella fastidiosa subsp. fastidiosa (Xf), responsible for the disease.
PIERCE’S DISEASE AT WORK
The pathogen targets the grapevine’s xylem vessels, which transport water throughout a plant, spreading the bacteria through the water channel and beyond. Glassy-winged sharpshooters, a leafhopper insect, feed on xylem sap, spreading the disease from one plant to another. The problem can escalate quickly.
“Both the PD bacterium and the leafhopper vectors have very wide host ranges including many weeds and shrubs,” Hopkins explains. “The bacteria move throughout the grapevine in the xylem vessels. High populations of the pathogen may completely block the water-conducting vessels, thus preventing water from getting from the roots to the branches and leaves of the plant.”
The results are devastating: scorched leaves, dried or wilted fruit, declined vigor, and even vine death. As soon as a plant shows symptoms, it needs to be removed before it can spread the disease to healthy plants. Growers sought preventative options had little recourse. Until recently, Hopkins says, plant resistance was the only long-term control, while other methods, like applying neonicotinoids or other systemic insecticides, were only marginally effective.
Today, there’s a promising solution on the horizon. Thanks to Hopkins, University of California (UC) Riverside collaborators Dr. Matt Daugherty and the late Dr. Bruce Kirkpatrick, and funding from California Department of Food and Agriculture’s Pierce’s Disease and Glassy-winged Sharpshooter Board, research and trials at UC Riverside have led to a new biological control that inoculates the plants against Pierce’s Disease.
The trials at the UC Riverside vineyard were designed to test whether a specific strain of Xylella fastidiosa, a plant xylem-colonizing bacterial pathogen that causes Pierce’s Disease, could prevent new plantings from becoming infected. The strain, EB92-1, is benign, meaning it is infectious to grapevines but won’t cause symptoms.
In July 2011, the first trial focused on ‘Cabernet Sauvignon’ and ‘Merlot’ vines. After being treated with the biological control at the UC Davis greenhouse, the vines were transplanted into the UC Riverside vineyard three months later. Due to the strong disease pressure and ample sharpshooter vectors at the California vineyard, Hopkins says, the location proved ideal for successfully evaluating the control’s effectiveness. The ‘Cabernet Sauvignon’ and ‘Merlot’ transplants grew into vigorous plants in the spring of 2012.
“The biocontrol strain provided excellent control of PD in ‘Cabernet Sauvignon’ and ‘Pinot Noir’ through 2017,” Hopkins says. “Over the six years of the trials, no treated vines have died; one treated ‘Pinot Noir’ vine had severe symptoms and would be removed from a commercial vineyard. In both cultivars, plant vigor and fruit production have remained very good.”
The UC Riverside tests aren’t the first to evaluate benign strains of Pierce’s Disease. Hopkins says there were greenhouse tests in 1992 that injected benign strains of the disease into plants. EB92-1 proved to be the most effective option, controlling the disease in Vitis vinifera (the most common type of cultivated grape) for 13 years in Central Florida, where the disease is widespread and persistent.
The positive results remain somewhat of a mystery, even to researchers.
“We do not know the exact mechanism of the biocontrol, but it appears to be the result of induced resistance in the plant,” Hopkins explains. “In a grapevine colonized by the biocontrol strain, the pathogenic strain does not grow well enough to block the xylem vessels and cause symptoms.”
As with all biological controls, there are some caveats, including that the treatment isn’t likely to be 100% effective. Hopkins says it may not be effective in previously infected plants. It works best in new plantings.
NEXT STEPS FOR THIS RESEARCH
For growers desperate to protect their grapevines, however, the EB92-1 strain offers a promising solution to a previously dire problem. Hopkins says there’s also a path to commercialization. Currently, the University of Florida owns the patent for the biocontrol bacterium and a commercial company is working to get it licensed for Pierce’s Disease. The registration process will take two to three years, but there’s light at the end of the tunnel.
In the meantime, growers can do their best to minimize the disease by using resistant cultivars. Hopkins recommends checking out the new options developed by Dr. Cheryl Walker at UC Davis. Systemic insecticides can help control the pathogen’s vectors and rouging out diseased plants can protect the rest of the crop. When the biocontrol becomes available to commercial growers, Hopkins is confident in its impact.
“This biocontrol has the potential to allow growers to successfully grow Vitis vinifera grapevines in areas where this is not now feasible, such as the Southeastern U.S.,” Hopkins says. “It also should reduce PD economic losses in areas like California that suffer because of the disease.”
Now that’s something worth raising a glass to.