UC Berkeley experts investigating why millions of trees in California have fallen victim to sudden oak death have figured out where the infestation began: Mount Tamalpais and Santa Cruz.
Scientists also now think the killer organism, which they suspect rode in on nonnative nursery plants, eventually was carried by humans to the two ground zero zones.
The new findings, disclosed Wednesday in San Rafael, add to the understanding of the biological mystery that has stumped scientists for more than a decade and has caused a swath of destruction in 14 counties, from Monterey to Humboldt. The disease has shown up in other countries, but the infection in California is considered by far the worst in the world.
Along with the huge number of fatalities, the disease has infected tens of millions of other trees.
"This is the most aggressive forest disease in the world, it is the No. 1 most wanted culprit," said researcher Matteo Garbelotto, one of the authors of the new study being published this month in Molecular Ecology. "It can travel around the world, it can wipe out hundreds of miles of forests. It is having a big impact in California - it is killing our favorite trees and disrupting the ecological network of our forests."
The disease was first reported in California in 1994 and the specific pathogen was identified six years later as a fungus-like organism known scientifically as Phytophthora ramorum. Experts believe that the pathogen arrived in the state through the "nursery trade," then spread outside. It is now the world's most quarantined plant pathogen.
The disease acquired its dramatic name because an infected tree can appear healthy for months even when it is actually dead. Then, abruptly, the tree's crown and canopy become brown; often, stricken oaks topple over. It infects about 100 species of plants, but the trees most affected are tan oaks, coast live oaks, California black oaks and canyon live oaks. It also infects redwoods, Douglas firs and bay laurels. They suffer leaf blight but don't die.
Reconstructing epidemic
"This new study is an important piece of research," said Jonathan Jones, who manages the sudden oak death program for the U.S. Department of Agriculture's Animal and Plant Health Inspection Service in Maryland. "To understand the population dynamic and the movement of the disease is very valuable."
In reconstructing the epidemic, researchers pinpointed the two forest sites where the disease was introduced in the wild and identified where it spread from those two points, more than 60 miles apart.
"Both sites make sense," said Garbelotto, a UC Berkeley associate extension specialist and adjunct professor. Details of his findings were disclosed Wednesday during a three-day gathering of the California Oak Mortality Task Force.
"Mount Tamalpais is near where sudden oak death was first reported, and the Santa Cruz site is near where the first nursery report was made," Garbelotto said. "The only explanation is that humans were involved, probably in the transportation of ornamental plants."
Researchers are searching for a weapon to fight the disease, but a solution is complicated because they've learned that as the organism has moved around the state, it genetically mutated - unique genotypes are surfacing in sites with new infestations.
In a twist, the scientists discovered that the bay laurel is often responsible for spreading the disease. Like an arboreal Typhoid Mary, the laurel may be slightly sickened, losing a branch or two, but otherwise not greatly affected. Yet it infects trees around it.
Rain assists disease spread
The disease spreads most readily when it rains. The pathogen produces a microscopic flask - guided by tiny propellers, the flask swims toward a new plant, and the infection begins anew. Using Global Positioning Systems, researchers determined the pathogen typically travels about 200 yards, but in a strong wind it can be transported as far as 3 miles.
"It is a lot further than we thought," Garbelotto said.
Big Sur has been hardest hit, suffering a higher level of tree mortality than Mount Tamalpais. More recently, western Marin and western Sonoma counties have been stricken.
Numerous countries have imposed strict regulations to control the spread of the disease. Phytophthora ramorum has been reported in 16 European Union countries, but relatively few trees have died. The pathogen has been discovered, as well, in the southwest corner of Oregon, where it is being successfully eradicated.
"California has the unfortunate perfect combination of host - tan oaks and bay laurels - and climate - cool, wet springs - and the organism is taking advantage of them," said Jones.
In California, a regulation controlling the movement of plants has been imposed on 14 counties since 2001. Plants can't be moved outside the regulated counties unless certified as free from the sudden oak death organism. The counties include San Francisco, San Mateo, Marin, Alameda, Contra Costa and Santa Cruz.
"Sudden oak disease hasn't moved into the Central Valley or to the Sierra. It has pretty much stayed in coastal areas," said Steve Lyle, a spokesman for the California Department of Food and Agriculture. "We hope that will remain the case."
About sudden oak death
What is it? Sudden oak death disease is a fungus-like pathogen - Phytophthora ramorum - that has killed more than a million trees in California.
How did it get here? Experts believe the organism was brought to California by infested ornamental plants, perhaps from Asia. Infected trees were transported to Mount Tamalpais and Santa Cruz, where the disease took off.
What does it do? An infected tree can appear healthy for months, even when dead, then abruptly turn brown.The San Francisco Chronicle
