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How oak death spores survive baffles scientists


Peter Fimrite, Chronicle Staff Writer
Friday, July 24, 2009

UC Berkeley researcher Catherine Eyre collects samples al...Rhododendron leaves are used to collect spores, then test...UC Berkeley researcher Catherine Eyre examines an oak tre...A petri dish holds collected leaf samples, which will be ...
The tree-strangling pathogen that causes sudden oak death is baffling scientists even after 14 years on its trail. The latest puzzle for sleuths in the field and laboratory is how the microscopic misfit manages to survive in waterways after all known sources and hosts have been removed.
Ten creeks, streams, drainage ditches and holding ponds in six states have been contaminated by the disease causing organism, known scientifically as Phytophthora ramorum, mostly as a result of irrigation runoff from nurseries, according to scientific papers released at a recent symposium. The mysterious pathogen has also been found in Crystal Springs Reservoir in San Mateo County and nearby drainages in the vicinity of infected and dying oak trees.
In several cases, the pathogen has persisted in the water, according to scientists, despite the fact that all runoff was halted, infected material was removed and the surrounding area was fumigated. The most troubling case is in Humboldt County, where two streams tested positive for spores tracked to a nearby nursery. The creek is at least a mile away from the nursery and there is no hydrological connection between them and no way for runoff to reach the stream.
"It is a completely baffling thing and it is very frustrating," said Yana Valachovic, a forester with the University of California Cooperative Extension Program out of Eureka.

Difficult to stop it:
The findings raise serious questions about how one can stop a microbial disease that gets more mysterious the more scientists learn about it. Millions of dollars in state and federal money have been spent trying to understand the disease and stop its spread, but the spores keep spreading and trees keep toppling. Sudden oak death, first discovered in Mill Valley in 1995, has killed tens of thousands of oak trees from Big Sur to southern Oregon. The pathogen infected hundreds of nurseries around the country in 2004 after diseased ornamental plants were shipped from Monrovia Growers in Azusa (Los Angeles County).

Spreads through water:
Arborists and ecologists are afraid that sudden oak death could eventually denude California's golden hills of its signature tree. It has been known for some time that Phytophthora ramorum spreads in water. After the 2004 Monrovia debacle, the U.S. Department of Agriculture's Animal and Plant Health Inspection Service ordered testing of water and waterways near infected nurseries. Since then the pathogen has been detected in waterways near nurseries in Florida, Georgia, Alabama, Mississippi, Washington, North and South Carolina. But it was the case in Humboldt County that stood out. Scientists were baffled in February when ramorum spores were discovered in Mill Creek in McKinleyville, 13 miles north of Eureka and 45 miles north of the nearest infested forest. Mill Creek is 2 miles south of Widow White Creek, where inoculum from the pathogen was detected in 2006 and 2007. Neither stream has any vegetation around it that is known to carry the pathogen.

Transfer a mystery:
Infected plants were found in a nursery between the two creeks in 2004, 2006 and 2007, but scientists cannot figure out how the spores got into the streams. Mill Creek is on a coastal bluff north of the Humboldt Bay watershed at least a mile away from the nursery. "There is some connection that I don't understand," Valachovic said. "The genetics match with the nursery, so it appears that it originated in that nursery. How it got to the stream is undetermined."
Researchers with the Forest Pathology and Mycology Laboratory at UC Berkeley are trying to find answers at the Crystal Springs Reservoir. Catherine Eyre, a post-doctoral researcher with the lab, said sporangia from the disease have been found in the water in at least half of the 14 sites tested this year in and around the reservoir, which supplies drinking water to millions of people in San Francisco and the Peninsula. "We're looking at how far the pathogen can travel and how long it can survive," said Eyre, adding that the spores are not dangerous to humans. "This is important because it could be one of the ways it is spreading around the state."
E-mail Peter Fimrite at

Hunting for sudden oak death disease

By Joe Rodriguez

Mercury News            5-11-09

When Roger Spreen went looking for the culprit killing Northern California's oak trees, he didn't have to look far. Sunday morning he nabbed a suspect lurking in his own backyard.

"That's a classic one right there," he said, grabbing it with one hand. Next he recorded its exact position using a GPS application on his cell phone.

The Los Altos Hills resident was one of 75 volunteers who had signed up for a "sod blitz" in and around the woodsy community. They were helping an ambitious project out of the University of California-Berkeley to pinpoint the flash points of sudden oak death syndrome.

This being epidemiological detective work, Spreen wasn't looking for the bodies of dead oaks but for evidence against the beautiful California bay tree, the prime carrier of the disease.

The furtive, runaway disease earned its name by killing oaks from the inside. After about a year, infected oaks succumb to the disease. Sudden oak disease has killed thousands of trees — most vulnerable is the coast live oak — in 14 counties from Monterey to Humboldt.

Spreen, a software programmer, and other volunteers were trained the day before by the project's leader, Matteo Garbelotto. a UC-Berkeley adjunct professor of forest pathology. With funding from the U.S. Department of Forestry, he's organized "sod blitzes'' from Carmel to Sonoma on weekends.

A walk with Spreen's blitz offered a look into how scientists trace the origins and spread of mysterious diseases, a handy thing to know after the recent swine flu outbreak. A diseased oak doesn't necessarily look sick.

"If they're infected, you wouldn't know," he said. "They're being eaten away from the inside, and they'll be dead in a year." If there is an advanced symptom, it's usually a tar-like liquid bleeding through the tree trunk.

Spreen was hunting for California bay trees carrying telltale signs of the disease.

"They grow like weeds," he said of the trees. "They're like the mosquitoes that carry malaria and pass it on to people."

He pointed to several bay trees growing around his property, which sits near Black Mountain, one of the tallest peaks in Silicon Valley. The majestic coast live oak, which dominates the area's hills and valleys, has become something of an attraction for homebuyers, cyclists and tourists. But if you look carefully, as Spreen pointed out, the number of bay trees growing next to the oaks is surprising.

"They can grow really tall and crowd out the oaks," he said. Encroachment, however, is not a problem in and of itself.

Somehow, the leaves of the bay tree became a host for the Phytophthora ramorum microbe — the pathogen, or germ, that causes sudden oak death syndrome. Scientists believe the microbe was introduced to California several years ago from foreign rhododendrons grown or sold in local nurseries.

Once the microbe finds a home on a bay tree leaf, usually during spring, it will swim downstream on rainwater to the tip of the longish leaf. Once there, it "burns" the leaf's tip, giving it the look of burned toast and an odd-looking yellow halo. When these leaves rub up against an oak, they can pass on the microbe, which then burrows into the oak's trunk and begins its dirty work. The bay trees, however, go on living unharmed.

Spreen, president of Los Altos Hills' open space committee, pointed to a neighbor's property. The homeowner, in an apparent panic, had removed every bay tree from his property.

That's not a solution prescribed by the California Oak Mortality Task Force, a nonprofit that brings together public agencies, researchers and communities in the fight against sudden oak death syndrome. In many cases, pruning the bay trees away from the oaks might be enough.

A native of Tennessee and graduate of Duke University, Spreen and his wife, Elizabeth, were attracted to the woodsy lifestyle of the mid-Peninsula hills.

"I still feel like a country boy from the South," Spreen said. After buying the property in 1996, the couple built an understated cabin-style house that blends softly into the landscape. Meanwhile, more than a few garish McMansions have popped up around them. While he can't do much about them, Spreen can at least try to protect the trees that give his community its charm as well as shade.

"For those of us who live this close to nature, trees are our landscaping," Spreen said. "You can cut down all your bay trees and still lose the oaks. Then you wouldn't have anything."

On his one day as a researcher, Spreen gathered at five suspicious leaves from each of eight bay trees around him and down the road. He dutifully jotted down their locations and tagged each tree with a green ribbon. Then he dropped them off at town hall, from where "sod blitz" drivers ran them to Garbelotto's lab.

Over the next three or four months, scientists there will test the leaves for the microbe and then prepare a map of the disease's spread. Depending on what they find, they hope to plan and launch a counterattack.

"I'm not a biologist and I don't even have a green thumb," Spreen said. "But this is something I can do as a citizen to help."

Contact Joe Rodriguez at 408-920-5767 or






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