Pathogen Outbreak Plagues Trees in Park

....Brice McPherson, research associate at the Center for Forestry on campus, noticed a particularly large outbreak of the disease in October 2006. Approximately 20 percent of the oaks in the park are affected.

The researchers, working with a $50,000 grant from East Bay Regional Parks that they obtained in August, began to study the Tilden oaks in November. So far, they have found the disease in every group of oaks they have inspected.

"Basically, the pathogen is a naturalized citizen of California," McPherson said. "It's everywhere within the forests where it has become established."

According to McPherson, the disease is caused when spores of a pathogenic water mold enter the tree's outer bark and attack the nutrient-conducting phloem tissue. The process attracts beetles that tunnel into the tree-the final cause of its death.

During its slow decay, which typically lasts two to five years, the infected oak oozes sap that resembles human blood from large cankers at its base, he said.

But what passers-by notice is the leaves' sudden change from green to brown immediately preceding the tree's death. For other plants, the disease has less severe effects.

"Most of the woody plants you see in Northern California are hosts for this pathogen, but for most, it is just a minor annoyance," McPherson said.

Outbreaks of Sudden Oak Death now plague Oregon, Washington, and British Columbia. All Bay Area counties are quarantine areas for the disease.

"We're trying to draw a line in the sand to stop the movement north," said Ronnie Eaton, deputy agricultural commissioner for Alameda County's Agricultural Commissioner's Office.

The commissioners inspect potential host plants before nurseries ship them outside the quarantine area and regulate the movement of susceptible plants and goods, like oak used for firewood.

Visitors of areas with high rates of infection are advised to clean their muddy shoes before leaving, Eaton said.

The water-borne pathogen also easily travels through streams, fog and wind, making containment of the disease difficult and posing serious problems for ecosystems.

"Our forests here have so many fewer tree species (than the East Coast), that the loss of oaks and tan oaks may lead to much more severe ecological impacts," McPherson stated in an e-mail.

He said many organisms depend on the oaks' acorns, since there are few other sources of high-quality plant protein.

While Oregon has used the slash-and-burn method to destroy areas where Sudden Oak Death has taken root, the problem's magnitude in California has precluded similar efforts.

"No one is under any eradication program whatsoever. It's just too far spread," said Joe Deviney, deputy agriculture commissioner for Contra Costa Country, which has jurisdiction over Tilden Park.

McPherson said it can be disheartening to study a disease with no apparent solution, but the work is still important.

"I don't really imagine there's anything anyone can do at this point," he said. "It's something we're going to have to come to terms with."

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