College of Natural Resources, UC Berkeley

Scientists Try Tricking Vineyard-Killing Bacteria

January 3, 2012

By Beth Mole, Santa Cruz Sentinel

Every grapevine in the 28-acre Bonny Doon Vineyard had to be ripped from the earth and torched in 1994. New vines might have faced the same fate the following year. Instead, owner Randall Grahm, numb from years of battling an incurable plague, sold his whole vineyard of dead and dying Syrah, Viognier and Marsanne grapes.

But that was just the beginning of a statewide killing spree by a new duo behind Pierce's disease: the sap-sucking insect known as the glassy-winged sharpshooter and the vine-choking bacteria Xylella fastidiosa. Together, they drained more than $30 million out of Northern California's $3 billion-a-year grape industry in the late '90s. The wine industry retaliated with millions of dollars of pest-management and protection measures -- in a battle it's still fighting.

Now, scientists have come up with a new and cheaper tactic: Confuse the germs as soon as the sharpshooter delivers them into a healthy vine. And it couldn't come at a better time for Grahm, who just bought land for a new vineyard in Bonny Doon.
Though Xylella and other sharpshooters have been in California since the 19th century, glassy-winged sharpshooters sneaked in during the 1980s, hopelessly infesting 12 southern counties warm enough to host them.

The strong fliers and heavy swarmers have shot up the state ever since, siphoning sap from plants along the way. Their crosshairs are on almonds, stone fruits and citrus. But grapes are where they cause the most damage.

Sapping the vines

They carry their partner in crime, Xylella, in their mouthparts and deliver it as they tap into a new vine. Once inside, the bacterium spreads through the plant's veins, and within a year clogs its plumbing so it can't get enough water.

"The unique thing about Pierce's disease is that it kills plants," said Larry Bettiga, a viticulture adviser at UC Davis. "You can't deal with that."

Grahm, speaking from experience, agreed: "Once the vines are infected, there's nothing you can do."

Every winery in Bonny Doon was affected in the mid-'90s. Some wine growers thought their time in California was up, Grahm said.

But by the late '90s, the California Department of Agriculture had launched full-scale pest-management programs that included spraying insecticides, monitoring sharpshooter activity, reaching vintners and farmers through outreach campaigns and inspecting all nursery plants shipped around the state -- the likely route glassy-winged sharpshooters used to get into California in the first place. Seventy percent of the state's nurseries are in areas infested with glassy-winged sharpshooters.

"Prevention is really critical," Bettiga said. "It's very difficult to eradicate an insect once it becomes established."

But the intensive pest-management program caused costs to soar.

Tricking the bacteria

So, scientists are now investigating less costly methods of managing the sharpshooters and the spread of Pierce's disease. Steve Lindow, a plant pathologist from UC Berkeley, is using something similar to a Jedi mind trick: Convince the bacteria they've already caused disease.

But to stop these microscopic killers, scientists had to do some criminal profiling.
When Xylella get into a grape vine, they're released in the vascular tissue -- the plumbing of the plant that pumps water up from the roots. From there, the bacteria use the tissue as "hallways" to invade the whole vine. They then start exploring and munching on the plant.

"We think that the exploratory phase involves rather promiscuous movement of bacteria," Lindow said. But as they spread from place to place, there are only a few bacteria in each area, he said.

This is key, he said, because when Xylella populations start to get big, they run into each other and switch tactics.

Each bacterium sends out a molecular beacon constantly, similar to the ping of a submarine's sonar. When many cells are stuck in part of the hallway at once, they're aware of each other because they can pick up the pings.

Then they mob up.

"It's kind of a switch -- a lifestyle switch," Lindow said. In a mob, they make themselves sticky -- to each other and to a spot -- so they can be sucked into a sharpshooter, their getaway car.

Restoring Bonny Doon

Lindow and his team of researchers realized that this beacon is the bacteria's glaring weakness -- without it, they wouldn't make it into their next sharpshooter or kill the vine.

So, the researchers engineered transgenic grape vines to make the same beacon.
If the vines constantly produce the signal for clumping, the arriving bacteria will get confused and act as though they've already infected the plant. They won't explore. They'll stay right where they landed and wait for their getaway insect -- even though they didn't pull off a heist.

Lindow thinks they'll start seeing results in fields in the next few years, but in the meantime, he'll also explore additional methods of treating Pierce's disease.
For Grahm, it's all good news for his effort to restore the essence of Bonny Doon wine.
"You can capture the distinctiveness of a site in the wine itself," he said. "Bonny Doon wines need to come from Bonny Doon."

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