Researchers Struggle to Save State’s Oak Trees

It’s being called appalling, heartbreaking and alarming.

Concerned homeowners, researchers and conservationists are warily watching an oak tree epidemic spread across coastal California. Entomologists, plant pathologists, ecologists, geographers and foresters are searching for answers.

Researchers cannot give exact numbers, but they are certain that thousands of tan oaks, coast live oaks and black oaks have died as a result of a disease complex termed “sudden oak death.”

Plant scientist David Rizzo from UC Davis and his colleagues on the UC Oak Research Team recently identified the culprit as a new species of fungus. They hope that the discovery will lead to a way to control or prevent the spread of the infection.

The new fungus is related to organisms that caused the Irish potato blight in the 1800s and the deaths of Port Orford cedars in Oregon--ongoing since 1938--and oak trees in Mexico in 1987.

“I did some DNA sequencing and it didn’t match any known species,” Rizzo said. “The [fungus] has been confirmed from Big Sur up through Santa Cruz and into Sonoma and southwestern Napa. Reports of symptoms have come from as far south as Santa Barbara and north to Humboldt. We’re working to confirm that it’s the same fungus” in those areas.

The fungus, from the genus Phytophthora, attacks trees by destroying the nutrient- and water-conducting tissues, causing bleeding of dark red sap.

Weakened by the fungus, trees succumb to infestation by the bark beetle, which may be attracted by the sap. This speeds up the death by completely choking off food and water.

The disease process can take from months to years. However, when a tree dies, its foliage quickly turns brown, giving the alarming appearance of a sudden death.

The fungus produces tiny, lemon-shaped spores that, when activated by water and cool temperatures, germinate into swimming spores that can infect the bark of the oaks. Once settled in the bark, the fungus secretes enzymes that digest the bark and the food-conducting cells. It feeds on the nutrients released by the digested cells.

In the lab, Rizzo found that the fungus produces swimming spores only at temperatures below 68 degrees Fahrenheit, which is also the optimal temperature for growth of the fungus.

Team members have begun to monitor the extent of the disease and death. Maggi Kelly, co-director of CAMFER, a center for forest assessment at UC Berkeley, is overseeing the two-part endeavor.

The first phase involves digital aerial photography of the affected areas--primarily Marin County--using both visible and near-infrared light. Kelly explained that when trees become stressed by disease, the leaf structure changes and reflects less light in the near-infrared spectrum, even though the leaves appear healthy.

It is hoped the images will allow researchers to distinguish between healthy trees and diseased and dead ones, which show up as gray patches in both types of light.

The second phase of monitoring tracks the progress of specific trees in 20 field plots in Marin County. Field researchers log the disease symptoms of 550 tagged trees every six to eight weeks. Aerial and field data will be linked, using Global Positioning System coordinates for each tree. Then researchers can determine if aerial data are reliable for assessing the rest of the state.

“All of this data we are gathering from the air, we can overlay with soil data and other geographic information and look for patterns,” Kelly said.

Brice McPherson, an insect ecologist, studies the beetle attacks that are ultimately killing the oaks. Although researchers believe that a fungal infection alone would mean the demise of an oak, the beetles are “faster and more efficient” at finishing off the trees, McPherson said.

McPherson would like to determine if the sap of the diseased trees attracts the beetles. “It’s fairly dramatic to see these bleeding patches and beetles associated with these patches,” McPherson said. According to one theory, a compound released by the oozing sap may mimic a pheromone that attracts the beetles.

The team is focused on discovering how the fungus spreads and preventing further spreading. Rizzo has proposed that spores are spread when wood or dirt are moved by logging, construction or nature enthusiasts. Spores might cling to boots and to tires of bicycles or other vehicles.

The U.S. Forest Service has requested a quarantine to stop the removal of infested oak stock and deadwood from affected areas. But Rizzo says it is too early to know if quarantines would be effective.

McPherson asked: “How would you stop it [in the wild]? I’m almost at a loss. If it’s spread through soil, bikes and boots, how do you stop that? Most of coastal California is overwhelmed by human beings who live here because they like hiking and biking.”

Rizzo cautioned that, for now, visitors to the affected coastal forest areas should wash off boots, pets’ feet, and car and bike tires before leaving the area. Also, firewood and soil should not be removed from the areas.

In April 1995, Pavel Svihra, a horticulturist with the UC cooperative extension, first saw the disease in Mill Valley. A homeowner had called when she noticed the sudden deaths of some tan oaks bordering her property.

Svihra now has the difficult task of advising the public on how to handle infected trees. Spraying a regulated pesticide called ASTRO has prolonged the lives of some trees, but Svihra says that will not solve the newly discovered fungus problem.

Svihra described the issue as “emotional” because the only other option for tree professionals is to cut down green, healthy-looking trees.

The California Oak Foundation advocates the protection of oak forests and its members are worried about the trees’ fate. Janet Santos Cobb, president of the foundation, said the oak deaths pose a major threat to watersheds, wildlife habitat and homes.

Under normal circumstances, live oak trees--thick-barked and green--are fire-resistant and actually slow the spread of wildfires. But McPherson described the numerous brown-leafed, dead trees as a wildfire waiting to happen.

The 30-plus-member Oak Research Team has been operating mainly on a $200,000 budget of emergency funds from the UC Division of Agriculture and Natural Resources and the U.S. Forest Service.

Experts agree that a solution will require more time and resources. There is a sense of urgency.

“I hate to be an alarmist but, at this stage of the game . . . this is a very big threat,” Rizzo said. “I’ll be very happy to say six months from now that I was wrong.

“Even though these oaks are not timber trees, the ecology is just tremendous: wildlife issues, fire issues. It’s pretty dramatic. . . . I would be worried.”

More information and a homeowner’s guide for diagnosing and treating infected oaks are available at https://cemarin.ucdavis.edu/.