Scientists finger California oak tree killer
Copyright 2000, Environmental News Network
August 9, 2000
By Sylvia Wright
Oak trees along the California coast are dying from a fungus that attacks tree bark. This oak forest, photographed June 20 in Miwok Meadows of China Camp State Park in California, shows the effects of the fungus.
A swimming, two-tailed fungus with an appetite for oak bark is the likely killer of thousands of trees along the coast of California, and scientists warn the deadly microbe could spread to other oak forests across the nation.
The fungi move around by spores that easily travel in infected wood and soil, and on bicycle tires, car tires, hikers' shoes and animals' feet, said David Rizzo, a plant pathologist at the University of California at Davis.
"Preventing the movement of soil and wood will be critical to slowing the spread of the fungus to other oak woodlands such as the Sierra Nevada," he said. "In particular, firewood and soil should not be moved from coastal areas."
Rizzo is a member of the University of California Oak Research Team assigned to track down and find a way to arrest the killer of thousands of oaks in the past five years from Santa Barbara to Humboldt counties. The team consists of research scientists from UC Davis and UC Berkeley, cooperative extension specialists and county farm advisers.
Rizzo's findings come on the eve of the region's rainy season, when the spores will be most abundant.
His investigation turned up a novel fungus related to organisms that caused the Irish potato famine of 1845-50. The fungus was also responsible for the modern-day deaths of Port Orford cedar trees in northern California and southern Oregon, eucalyptus forests in Australia and oak forests in Mexico, Spain and Portugal.
Oozing occurs on the trunk of an oak tree stricken by the fungus.
The killer is a member of the genus Phytophthora. The species has not been named or even fully described, but scientists know it does not match any of the 60 known Phytophthora species anywhere in the world, according to Rizzo.
It is not known if the fungus was recently introduced to California or is native to the area. Contrary to some reports, the pathogen is not related to the oak wilt fungus (Ceratocystis fagacearum), a significant oak tree disease in the eastern United States.
The fungus Rizzo isolated likes the cool, wet conditions typically found along much of the California coastline, especially in the redwood forests of the foggy coastal ranges.
"It cannot be seen unaided, but under a microscope, colonies of the fungus look like clusters of cotton fibers," he said.
These little creatures are highly fecund, Rizzo discovered. A single organism kept at 59 to 68 degrees Fahrenheit can produce thousands of offspring in 24 hours.
Those offspring, called zoospores, are released from little lemon-shaped sacs named sporangia.
"It appears the spores don't enter through the tree roots, as many Phytophthora species do. Instead, they move through the bark on tree trunks and limbs, possibly with the help of raindrops," said Rizzo.
Once in the tree, the fungus produces enzymes that dissolve the dead outer and living inner layers of bark. Oozing sores result as the cell walls break down.
As the disease progresses past the bark and into the wood, the tree becomes so weak that it is vulnerable to bark beetles, which burrow into the tree and kill it by blocking its circulatory system.
Once Rizzo isolated the fungus, he found it in tissue samples from California coast live oak, black oak and tanoak trees in an area from Big Sur to Sonoma County to southwestern Napa County. It has not been found in other oaks such as the valley oak, blue oak and interior live oak.
Black oaks are the most common oaks at middle to high elevations in the Sierra Nevada such as in Yosemite Valley.