Pierce's disease has plagued California's vineyards for more than a century, slowly killing vines and costing the industry millions of dollars a year. There is no known cure. Now a team of researchers from the University of California at Davis has found that even our understanding of how the bacteria behind the disease kills grapevines is incorrect. While that may sound frustrating, their new theory could help lead to a long-awaited cure.
Xylella fastidiosa is a bacteria transmitted to vines by the bite of a group of insects known as sharpshooters. For grapevines, infection is a death sentence.
Until now, scientists believed that a gel-like substance produced inside the vine in response to xylella clogs the plant's xylem tissue, cutting off the circulation of vital moisture and nutrients to leaves, ultimately "choking" the vine as leaves turn yellowish brown and then die. Treatments have been aimed at killing the bacteria, eradicating the sharpshooters that carry xylella, breeding vines that are immune to the disease and even using viruses that attack xylella.
But according to Dr. Abaya Dandekar, a plant scientist at U.C. Davis, new research suggests that an enzyme (dubbed LesA) produced by the xylella may be responsible for the characteristic vine damage and death. Dandekar and his colleagues observed that some of the leaves showing the most dramatic symptoms of Pierce's often contained very low concentrations of xylella, while some leaves showing very mild symptoms had high concentrations. They also observed that leaves with the worst symptoms usually had the highest concentrations of LesA in them.
To test their theory, graduate students Rafael Nascimiento and Hossein Gouran manipulated xylella's genes, knocking out the gene that produces the LesA enzyme. The result? Vines infected with the altered form of xylella showed little or no signs of the disease or its resulting damage.
Dandekar has hypothesized that the LesA enzyme may play a role in the breakdown of cell walls, leaving residues that provide nourishment for the xylella, and allowing it to proliferate. That attack on cell walls is what causes the disease symptoms and death of the vine. "If we take this protein [LesA] and inject it into plants, we see these scorching-like symptoms," said Dandekar.
The team's discovery, while offering no immediate cure for the disease, opens new doors for research. "We don't really have any way to treat this disease at present. All we do is target its insect vectors," said Dandekar. "This new research helps us understand how we might ultimately eradicate this bacteria."
While the hunt for a cure continues, vintners in several California regions are still grappling with Pierce’s disease. Napa and Sonoma counties both experienced increases in Pierce's in 2015. Now growers in the Central Valley, source of the vast majority of America's value wines and raisin grapes, are reporting a frightening upsurge.
Agriculture officials and grapegrowers in Kern County in the San Joaquin Valley are reporting record numbers of glassy-winged sharpshooters trapped last season, more than in any single year since serious outbreaks began in 2001 with the insect’s arrival in the area. With them comes an increase in visible symptoms of Pierce's.
Kern County has more than 67,000 acres of vines, of which about 18,000 acres are planted with wine grape varieties. The area also contains large citrus tree groves and eucalyptus trees planted as windbreaks. Both are known to be favorite spots for the bugs to spend winters.
In the past, cold and foggy winter conditions have kept the insects' population in check, but the last two years have seen little or no fog and higher average temperatures, allowing the sharpshooters to feed and reproduce during the winter months.
"When I was a kid, it used to be a lot foggier, and the cold keeps the sharpshooter populations down," said Judy Zaninovich, a member of a Pierce's task force for the California Department of Food and Agriculture who grew up in the area. "The insect will die after three or four days at 55° F or below."
Another problem: a lack of cooperation among some growers with the task force's efforts to control the problem using chemical treatments and vine removal. While most growers have been cooperative, a few have not. At a January meeting of the task force board, members were shown photos of one neglected vineyard in the area, full of weeds and Pierce’s-infected vines. The property has been effectively serving as a reservoir of the disease, imperiling nearby vineyards.
"We've tried working with this grower for two to three years, but we've reached a standstill," said Glenn Fankhauser, Kern County's assistant agricultural commissioner. "Evidently this person doesn't understand that their own vines will eventually self-destruct if they don't keep up with sanitation."
The vineyard owner could not be reached for comment. Officials are currently working with the local district attorney to pursue court action to force the removal of the infested vineyard.
While cooler temperatures this winter may offer some hope for greater control of the sharpshooter population, growers and officials are planning ways to control the spread of both the bug and the disease.