Mysterious Disease Is Killing Oaks
Steps away from the shaded hush of redwood groves here on Mt. Tamalpais, another symbol of California is dying.
An unknown disease is swiftly killing the tanbark oak, a tree once prized by California Indians for its acorns yet scorned by foresters as a weed. Dubbed “sudden death,” the mysterious illness has stumped scientists even as it has spread to neighboring counties and other types of trees at near-epidemic rates.
“Yes, I’d say you could call it an epidemic,” said Rick Standiford, associate dean of forestry at UC Berkeley. “The range now reaches from Monterey to Mendocino. In stands of trees we’re studying, 48% are showing at least some symptoms.”
Sudden death, described as a disease complex because no single cause, or pathogen, has been isolated, was first identified in Muir Woods and the nearby Marin County town of Mill Valley in the mid-1990s. First affected were tanoaks, an evergreen species of the beech-oak family, a cousin to the oak tree. Coast live oaks and California black oaks soon succumbed.
Thousands of trees have been affected, leaving swaths of brown leaves and bare branches in the Central Coast’s rolling hills. Recently, signs of the disease have appeared in the California canyon oak, whose gnarled limbs and graceful canopies have come to symbolize open space in Southern California.
“There has been some perception this is related to the weather and will go away,” Standiford said, referring to record rainfalls from El Nino-fueled storms. “But in my mind, there’s no question there’s something going, whether it’s fungal, bacterial or a virus.”
The problem came to light in 1995, when a group of Mill Valley homeowners asked for help after dozens of tanoaks on their property suddenly died. Pavel Svihra, a horticulturist with the California Cooperative Extension Service, first diagnosed a common oak root fungus, but he soon changed his mind.
“Normally, when we have destructive diseases on trees they always begin with a branch or limb and then it spreads, but this involves the whole tree rather than just a limb or the roots,” Svihra said. “It’s very mysterious and we still don’t understand it at all.”
Also unusual is the pace of the trees’ decline. Tree death from all but the most virulent strains of fungus takes one to two years. The tanoaks and California oaks die in a matter of weeks.
Symptoms follow a pattern. The young shoot tips of new branches wilt, leaves and twigs die and a dark red sap bleeds from the lower portion of the trunk. Dead leaves stay on the trees for as long as six months, then drop. New suckers grow from the trunks of some trees, but fail to survive.
While the oaks are in this weakened state, tiny ambrosia and oak bark beetles infest the trees. Horticulturists advise homeowners to cut and dispose of the afflicted oaks, and tree services throughout the Bay Area report dozens of such calls each week.
Despite the scope and speed of the oak deaths, foresters and arborists paid little attention to the disease for the first few years, Svihra said. Tanoaks, once a major source of tannin used for processing leather, are considered trash trees whose crooked growth and porous wood grain hold little commercial value.
The trees were revered by American Indian tribes in the region, who leached the bitter tannin from the large and abundant acorns and ground them into flour. The trees also provide habitat for more than 300 species of animals and more than 5,000 types of insects, conservationists say.
“It was quite frustrating. I thought that this would be explosive information, but because it affected the tanoak, there was not much excitement,” Svihra said. “In 1998, when suddenly the live oaks were hit, people became alarmed as large numbers of these precious oaks were disappearing from their gardens.”
The disease poses a serious threat in forests as well, said Nicole Palkovsky, coordinator of the Oak Project, a team of researchers, arborists and community leaders studying the sudden death syndrome.
“Oaks are really important in the landscape in terms of wildlife,” she said. ‘The live trees not only provide food and habitat, but the large numbers of dead trees present a huge fire hazard.”
Researchers are studying the disease in the forests, in the laboratory and from the air. They have targeted stands of trees throughout the region and, using aerial photographs, calculate the speed and spread of the disease. Slices of affected trees have been sent to several labs for testing in an effort to isolate a fungus, bacterium or virus.
Some suspect the cause to be an imported pathogen, similar to the nonnative agents that destroyed the nation’s chestnut and elm trees. Scientists recently traced the fungus that all but wiped out the American chestnut tree to a source in Japan. Researchers believe that Dutch elm disease, a plague that destroyed 60% of the nation’s elms, was carried on a single log imported from New York to the Midwest.
Whatever the cause of the oak disease, the scientific community has rallied. The University of California has given the Oak Tree Project $50,000, and later this month Standiford will meet with the U.S. Forest Service about a possible $70,000 grant. Private foundations and state agencies are also being tapped for funds.
“Every plant is a source of information and has intrinsic value in and of themselves,” said David Chipping, vice president of conservation for the California Native Plant Society. “These oaks are of great value in the ecosystem, and it is terrible that we are losing them.”
