Fungal Disease Is Killing Oak Trees in Parts of U.S.

By John Roach
for National Geographic News
December 6, 2001
Scientists are in a race against time to stifle the spread of a fungal
disease that has killed tens of thousands of stately oak trees in
California and southwestern Oregon. Many people fear it may spread and
similarly wipe out large swaths of forest in the eastern United States
and Canada.

The disease, called sudden oak death, was first
detected in 1955 in California tan oaks. Since then, it has killed more
than 100,000 tan oaks, coast live oaks, black oaks, and Shreve's oaks in
coastal regions of northern California, and more recently in
southwestern Oregon.

In August, scientists at the University of California at Berkeley announced results from a laboratory experiment that showed eastern oak species, such as pin and red oaks, are also susceptible to the disease.

"We need to be worried about it, we need to be cognitive of it, and we need to try to stop its spread," said David Rizzo, a plant pathologist at the University of California. He presented his research about the potential reach of sudden oak death at the recent annual meeting of the American Phytopathological Society in Salt Lake City, Utah.

If the disease is not controlled, scientists fear that oak trees could go the way of the American chestnut tree, which dominated forests in the eastern United States until they were virtually wiped out by a fungus in the early part of the 20th century.

Unknown Origin

The oak disease is caused by a recently named fungal organism called Phytophthora ramorum. It also attacks a dozen other tree and plant species, including rhododendron, huckleberry, honeysuckle, coffeeberry, manzanita, buckeye, big leaf maple, bay laurel, evergreen, and madrone.

The pathogen is of the same genus responsible for the Irish potato famine in the mid-1800s. Rizzo and his colleagues speculate that the pathogen came to the United States from Germany, where it has infected rhododendrons since 1993. However, test results have yet to confirm this speculation.

"There is no natural resistance to the pathogen. It can romp right through a forest, much like the way Europeans brought smallpox [to North America] and killed more Native Americans with disease than they did by any other means," said Rizzo.

In places such as Muir Woods National Monument, just north of San Francisco, sudden oak death has killed nearly all the native oak trees that were once interspersed amongst the towering redwood trees the monument was established to protect.

Preliminary tests on the disease indicate that it enters through the bark of plants and trees, rather than the roots, as many Phytophthora species do, and travels in raindrops.

How sudden oak death has spread hundreds of miles up and down the coast, killing most of the oak trees and rhododendron plants in its path, has eluded the scientists so far.

"The pathways are not clear, but humans will be one of them," said Rizzo. People could spread the disease through a variety of means, such as in dirt on tires or hiking boots, or from pieces of firewood from a camping trip that have been left in the back of a car.

The scientists believe the pathogen can be carried long distances by the wind and birds. This may explain how the disease appeared in August in a remote section of forest in southwestern Oregon even though affected California counties had been quarantined to curb the spread of the disease.

Curbing the Spread

Government agencies in California and Oregon have spent millions of dollars and thousands of hours of labor to stifle the further spread of sudden oak death. In Oregon, where the disease was recently introduced, efforts are underway to cut down and burn all host plants in infected areas.

"If the fungus were to remain up in the tops of the trees, storms could very well blow it around and increase chances that it would spread," Alan Kanaskie, a forest pathologist at the Oregon Department of Forestry said in a statement about control efforts.

Oregon, along with Canada and South Korea, have implemented quarantines on oak products and all other hosts of the disease from California.

Meanwhile, the federal government and the state of California have allocated millions of dollars to combat the disease. Some of the money is funding research efforts like those led by Rizzo. Laboratory experiments at the Davis and Berkeley campuses of the University of California indicate that certain chemicals (phosphites) injected through small holes drilled into the bark of a tree can slow the progress of the disease, but nothing yet has been found to eliminate sudden oak death.

"The disease is here to stay," said Rizzo. "We just hope to prevent its spread to other areas."

© 1996-2008 National Geographic Society. All rights reserved.