Although Pierce’s disease has been a threat to California vineyards since at least the 1880s, when it wiped out 40,000 acres of wine grapes in the Los Angeles Basin, there’s still no cure for it. However, Don Hopkins, a University of Florida plant pathologist, has found a way to protect uninfected vines from the disease by giving them a shot of a benign strain of the same type of bacterium that causes it.
In an extensive series of lab studies and in field trials at vineyards in several states including those at the University of California – Riverside, this form of biocontrol has proven effective in preventing vines from developing the disease, Hopkins said.
This benign bacterial strain is part of a broader class of control agents often referred to as symbiotic control. The strain was patented in 2009 by the University of Florida, which recently licensed it for commercial development.
In Florida, where vineyards have long faced heavy pressure from Pierce’s disease, Vitis vinifera grape vines typically die within about three or four years after becoming infected with the disease. However, one block of Cabernet Sauvignon in a Florida vineyard remains free of Pierce’s disease symptoms 15 years after Hopkins inoculated the vines with the benign strain of the pathogen.
The disease is caused by the bacterium, Xylella fastidiosa. Growing in the xylem, the water-conducting vessels of the vine, it can kill grape vines in as little as a year or two by blocking movement of the water.
The economic cost of Pierce’s disease to California’s grape growers began rising significantly in the early 1990s, not long after arrival of the invasive glassy-winged sharpshooter, likely as eggs on nursery stock from the southeastern United States. It’s one of several members from two groups of insects – sharpshooters and spittlebugs – which can spread the disease by feeding on the xylem sap.
The glassy-winged sharpshooter is the most threatening vector because of its ability to use a much greater diversity of host plants – more than 300 different types – than the others and to achieve much higher population densities.
The glassy-winged sharpshooter was first found in Orange and Ventura counties in 1989. Since then it has spread throughout southern California and into the central part of the state, including San Luis Obispo, Santa Barbara, Santa Clara and Tulare counties. Recently, after several years of effectively controlling the glassy-winged sharpshooter, growers in some areas of the state have seen populations of the pest beginning to resurge significantly.
Hopkins discovered the benign strain of Xylella fastidiosa in an elderberry bush as part of his initial research on the bacterium. During several years of lab testing in the early 1990s, he found that, by introducing this benign bacterial strain into very young vines and then exposing them to the pathogenic strain, most vines did not develop Pierce’s disease.
In the lab, Hopkins treated the very young vines by placing a drop of the benign bacterial suspension on the stems. Then, he punctured the vines with a needle, which allowed the bacteria to be taken up in the xylem. Two weeks later he used this approach to inject these same vines with the pathogenic strain. In most cases, the vines treated with the biocontrol did not show symptoms of Pierce’s disease.
Meanwhile, others vines, which did not receive the biocontrol treatment, were injected with the pathogen. Most of them later developed Pierce’s disease.
Hopkins then repeated these trials under actual field conditions in a Florida vineyard. “The biocontrol turned out to be more effective at preventing the disease in the vineyard than it was in the greenhouse,” Hopkins says.
Next followed vineyard trials involving a number of different wine grape varieties in Florida as well as in Georgia and California. Vines that were not inoculated with the benign strain of the bacterium often developed symptoms of Pierce’s disease in one to six months.
“Usually, in new plantings, we can see a difference in the health of treated and non-treated vines within from six months to three years,” Hopkins says.
Meanwhile, most of the inoculated vines remained free of the disease throughout the five- to six-year-length of the studies.
“We don’t know how this benign strain of the bacterium works to protect the vine,” he says. “Somehow it doesn’t allow the pathogen to build up in large enough numbers to block the xylem.”
Results of the UC-Riverside vineyard trials with Cabernet Sauvignon and Pinot Noir, which began in 2012, have been similar to those in Florida and Georgia.
“As of last year, we had not lost any treated vines to Pierce’s disease, and the growth, vigor and production of most of these vines remained really good,” Hopkins said. “However, several untreated vines died from the disease. Also, some of the treated vines were showing symptoms of the disease by the end of the trial. Either they didn’t get inoculated properly or the pathogen overcame the benign strain of the bacterium. But, that’s to be expected. As with any type of biocontrol, treatment is not 100 percent effective.”
Hopkins notes some of the other findings in this research:
- Early on he and his colleagues noticed the benign bacterium doesn’t colonize young vines much past the 10th or 12th internode. The colony of the pathogenic strain, on the other hand, continues to advance through the plant as the vine grows. Still, the biocontrol induces resistance to Pierce’s disease beyond the area which it colonizes.
- The benign strain of the bacterium multiplies at a much lower rate than the pathogenic strain. In fact, Hopkins notes, the population of the strain of benign bacterium in the vine is 100-fold smaller than the pathogenic strain and does not build up populations in the grape plant that will cause symptoms.
Hopkins says that, while this treatment can prevent most of the vineyard from becoming infected with Pierce’s disease, it won’t cure vines that have already been infected with it. “If a mature grape vine is coming down with Pierce’s disease, this biocontrol treatment is not going to prevent the disease from developing and, eventually, killing the vine,” he says.
While stress can make Pierce’s disease symptoms more severe, the benign strain has provided control compared to the untreated in all conditions, including drought, other diseases, and insect pressure.
Ideally, the best use of this benign bacterium would be to treat very young vines while they’re still in a greenhouse or transplant house before they are transplanted in the field, Hopkins says. Inoculating the small individual vines while on benches is much easier than in a vineyard. Plus, it minimizes the risk of newly planted vines from contracting Pierce’s disease before they are treated.
The ability of this benign form of the bacterium to protect vines for the long term are promising, he adds.
“Originally, we thought vines would have to be inoculated with the biocontrol every year or two for the treatment to remain effective,” Hopkins says. “But, we’ve been surprised. Some of the data indicate that one treatment might protect the vines for life. Normally, as long as a biocontrol strain survives, it continues to provide good protection. In this case, one treatment and, maybe, a booster later, should be enough to prevent infection with Pierce’s disease.”