A swimming, two-tailed fungus with an appetite for oak bark is probably to blame for the death of thousands of trees in coastal California, and if people aren't careful, the deadly microbe could spread to the nation's other oak forests, a UC Davis plant scientist said today.
The fungi move around by spores that can easily travel in infected wood and soil, on bicycle and car tires, hikers' shoes and animals' feet, said UC Davis plant pathologist David Rizzo.
Rizzo is a member of the University of California Oak Research Team that was assigned to track down and understand 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 advisors.
Rizzo said he released his findings today in an attempt to raise public consciousness before the start of the rainy season, when the spores will be most abundant.
"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." Any wood already moved elsewhere should be burned.
Visitors to coastal forests should clean their tires, shoes and animals' feet thoroughly before leaving the area. Construction workers should wash equipment well and should not move dirt from one place to another.
Some infected oak trees may be able to be treated and saved, Rizzo said, but property owners should first be sure to have a sick tree diagnosed before beginning any treatment.
Rizzo's investigation showed that the culprit is a novel fungus related to the organisms that caused the Irish potato famine of 1845-50 and the modern deaths of Port Orford cedar trees in Northern California and southern Oregon, eucalyptus forests in Australia and oak forests in Mexico, Spain and Portugal.
It is a member of the genus Phytophthora (pronounced Phy-TOFF-thor-uh); the species has not been named or even fully described yet. It does not match any of the 60 known Phytophthora species anywhere in the world, Rizzo said. It is not known if the fungus has been recently introduced into California or is native and has only recently increased in importance as a tree-killer. Contrary to some Internet reports, the pathogen is not related to the oak wilt fungus (Ceratocystis fagacearum), an important disease of oaks 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 coast ranges. It cannot be seen unaided but under a microscope, colonies of the fungus look like clusters of cotton fibers, he said. Those little creatures are highly fecund, he found; 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, Rizzo said. He and his colleagues still have a lot to learn about the spread of the spores and the way they damage the oak tree, but it appears that the spores don't enter through the tree roots, as many Phytophthora species do. Instead, they enter through the bark on tree trunks and limbs, possibly after they are splashed there by raindrops, he said. They have two propellant tails, called flagella, that send them swimming quickly through any water.
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 coast live oak, black oak and tanoak trees in an area from Big Sur to Sonoma County and southwestern Napa County. It has not been found in other oaks such as 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.
For more information about diagnosis and treatment, visit the University of California Cooperative Extension in Marin County web site at http://cemarin.ucdavis.edu/index2.html
Add'l media contact:
David Rizzo, plant pathology, 530-754-9255, dmrizzo@ucdavis.edu