Experimental Solution to Pierce's Disease

Texas researchers develop phage-based cocktail of biocontrols for major grapevine malady

by Linda Jones McKee
pierce's disease phage
Dr. Mayukh Das from Texas A&M AgriLife's Center for Phage Technology conducts Pierce's disease research. Photo: Texas A&M AgriLife Research
College Station, Texas—For decades, Pierce’s disease (PD) prevented wineries from growing vinifera grapes across the southeastern United States—from North Carolina to Texas—and was also found in California. Caused by the bacterium Xylella fastidiosa (Xf), PD is spread by a number of different vectors, including the glassy-winged sharpshooter insect (GWSS).

Rather than increasing the resistance level of grapevines to PD or zapping the vectors spreading it, Dr. Carlos Gonzalez, professor of plant bacteriology at Texas A&M, and his team of researchers developed a biocontrol system using bacteriophages to go after the bacterium itself. A bacteriophage, or phage, is a virus that attacks bacteria, and in this case, Gonzalez’ team isolated phages that target Xylella fastidiosa. They used a phage “cocktail” that included four different phages in order to make it effective against PD, even if the Xf became resistant to one of the phages.

Finding a way to control or eradicate PD became a major concern when GWSS, a non-native sharpshooter, was found in southern California in the early 1990s. By 1999, approximately 300 acres of grapes in the Temecula Valley were identified as having PD, and in the following three years, more than 1,100 acres of vineyard had been killed by PD across the state. It was quickly evident that this disease could have a major impact on California’s wine and grape industry.

Since GWSS was found in California, millions of dollars have been spent on research projects and various programs by local, state and federal agencies in the attempt to find solutions to PD, to control or eliminate the spread of the GWSS, and to regulate the shipments of grapevine stock from nurseries in infested areas to other parts of California.

On one front, Dr. Andrew Walker, professor of viticulture and enology at the University of California, Davis, has had good success in his project to breed grapevines with resistance to Pierce’s disease. Using genetic mapping tools and aggressive growing techniques, during the past 12 years Walker has produced vines that are 97% vinifera, produce good quality wine and are resistant to PD. However, it will still be several years before PD-resistant vines are commercially available.

Historically, growers have used insecticides such as Admire to control the disease-spreading insect vectors such as GWSS. “It’s critical to get other solutions,” Dr. David N. Appel, professor of plant pathology and Texas A&M AgriLife extension specialist, told Wines & Vines. “The primary ingredient in these insecticides is Imidacloprid, which has been banned in Europe because of its effect on the bee population.” What insecticides the Environmental Protection Agency will allow to be used in the United States is always a question. And increasingly, the American public is concerned about the use of pesticides in any agricultural setting.

Bacteriophages, however, offer a “green” alternative to pesticides because they are abundant in the environment and target specific bacteria. Drs. Stephen Ahern, Mayukh Das, Tushar Suvra Bhowmick and Ry Young worked with Gonzalez to isolate phages and determine their genetic diversity. The team has a phage bank with more than 100 phages, and from that they selected four virulent phages (known to the researchers as Sano, Salvo, Prado and Paz) based on their physical and genetic characteristics. Well-developed phage cocktails can act as a natural biocontrol, targeting a pathogenic bacterium without affecting humans, animals or plants or their associated microflora.

The phage cocktail was then tested on grapevines grown in a greenhouse. Some vines were inoculated with the cocktail before they were infected with the Xf bacterium, and others were infected first with Xf and then treated with the cocktail. Results showed that the phage cocktail worked to prevent disease development and stop the disease when it was already present. “We found that we can reduce the bacterium to where there is no disease in greenhouse experiments,” Gonzalez stated. “A phage is the only ‘medicine’ that replicates in the presence of the host. In vines treated with the cocktail prophylactically, the phages will last up to 12 weeks without a host.”

Field trials are currently being conducted in Texas. “We’re further along in Texas but don’t have final results,” Gonzalez commented. “We need an entire growing season, and then a second season, partly so we can check plant vigor.” In the vineyard, vines are injected with the cocktail using a special injection gun. The research team took an injection gun designed for animals and modified it to inject plants at a rate of 200 plants per hour. The prototype gun was quite expensive, but guns for growers cost about $4,500. When questioned about scaling the inoculation rate for large vineyards, Gonzalez replied, “Growers may not have to inoculate every plant. Good growers ‘pre-mitigate’ by taking care of things, such as removing riparian areas to help knock back the vectors that are known to live there.”

Additional field trials are planned in California next year. The planning process has been completed for a statistically valid trial with all treatments and controls. “We’ll need about two years of field data before we can apply for EPA approval as a biopesticide,” Gonzalez added. The goal is to develop a sustainable phage treatment that is environmentally friendly and effective.

“This is still experimental,” Gonzalez emphasized. “It will take two and a half to three years for it to be available to growers.” It is not only grapegrowers who are waiting for this research to be translated into practical application. Xf also causes diseases in other plants such as peaches, plums, almonds, olives, citrus and coffee, and this research may have use in these crops as well. Gonzalez noted that in the laboratory, Xf strains isolated from diseased almond, olive (from the United States) and coffee plants were sensitive to the phages in their collection. 

Posted on 08.13.2015 - 12:14:47 PST
Congratulations to the team! It is amazing what can be done with biological controls. Keep adding to your cocktail with additional beneficial organisms, after all native soils are a plethora of beneficial biology looking for company.
Bruce Coulthard