Midpen staff and California Conservation Corps members remove California bay trees from the Los Trancos Preserve in Los Altos, in order to protect nearby oak trees from the fungus-like pathogen that causes Sudden Oak Death. (Courtesy Midpeninsula Regional Open Space District)

Scientists study incurable tree disease in San Mateo County

There is nothing sudden about stopping Sudden Oak Death.

But a group of scientists from around the world still plan to study later this month in San Mateo County the incurable disease that kills oak trees.

Sudden Oak Death is threatening forests in coastal California counties from Humboldt to Monterey. Because the plant disease has established a foothold in the Bay Area, San Francisco was a natural choice for the Sixth Sudden Oak Death Science Symposium.

The Midpeninsula Regional Open Space District (Midpen) will host the scientists for a tour of some of the San Mateo County preserves that have been impacted by the disease. The field trip is not available to the public.

Sudden Oak Death is caused by Phytophthora ramorum, a fungus-like pathogen that may have originated in Asian forests, Midpen Senior Resource Management Specialist Cindy Roessler explained.

A type of water mold, the disease is believed to have been introduced to Northern California after it escaped from commercial nurseries, Roessler said. It was first noticed in the wild in the Bay Area around the year 2000.

Phytophthora ramorum has made inroads along the coast because warm, wet springs provide the ideal environment for the organism to spread, Roessler explained. But while the fungus-like parasite has managed to establish itself in the Bay Area, Roessler said the California drought has slowed the spread of the disease for the last four years.

An aerial survey of counties around the Bay Area and Monterey Bay by the U.S. Forest Service revealed 15,600 acres worth of trees had died from Sudden Oak Death, Roessler said. The extent of the damage in San Mateo County is not yet known.

Midpen is responsible for 60,000 acres of land that’s divided into 24 preserves, only five of which have been unaffected by the disease.

But Midpen has been fighting back against the parasite for more than a decade, collaborating with other organizations on studying and implementing mitigation efforts, Roessler said.

“We decided 10 years ago that instead of just watching the devastation, we were going to do something about it,” Roessler said. “But it takes a long time to understand these kinds of diseases and know how to respond.”

Midpen has also partnered with UC Berkeley to find solutions, including an effort to find tanoak trees that are naturally resistant to the disease.

Tanoak, or tanbark-oak, is normally one of the oak species most vulnerable to Sudden Oak Death, Roessler said, but some are resistant. If seeds from resistant tanoaks can be identified, their trees could be planted in places where less resistant tanoaks have already succumbed, the scientist noted.

“Our volunteers have been out there collecting acorns like little squirrels,” Roessler said. “People love oak trees, and they want to do something.”

In another effort, Midpen has been looking at the role the California bay tree (also known as the California laurel) plays in spreading the disease to oaks.

Phytophthora ramorum does not get inside bay trees’ living wood, as it does with oak trees. Bay trees, therefore, do not die from being infected, but the organism can grow on bay tree leaves and use them as jumping-off points for launching spores.

“We call the California bay tree the Typhoid Mary of Sudden Oak Death,” Roessler noted.

And removing all bay trees within 15 feet of a given oak seems to improve that tree’s chances of survival, Roessler said, because the Phytophthora ramorum spores are less able to reach it.

Therefore, Midpen partnered with the California Conservation Corps, whose youth work trainees cleared bay trees away from a test group of 500 oaks.

Those oaks already appear to have a “slightly” increased survival rate, Roessler said. Midpen scientists will be keeping a close eye on them in the years to come.

The symposium is June 21-23 at Fort Mason.

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