Scripps Howard News Service

SAN FRANCISCO -- University of California, 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, later was carried by humans to the two ground zero zones.

The new findings 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 California counties. While the disease has shown up in other countries, the infection in California is considered by far the worst in the world.

"This is the most aggressive forest disease in the world. It, 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.

Trees infected with the disease can appear healthy for months even when it they are actually dead. Then, abruptly, the tree's crown and canopy become brown. Often, they topple over. Most of the victims are tan oaks, coast live oaks, California black oaks and canyon live oaks. Redwoods, Douglas firs and bay laurels suffer leaf blight from the disease but don't die.

"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 then spread from, more than 60 miles beyond.

"Both sites make sense," said Garbelotto, a UC Berkeley associate extension specialist and adjunct professor.

"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.

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.

Numerous countries have imposed strict regulations to control the spread of the disease, which has been reported in 16 European Union countries, where tree deaths have been rare. 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.

(Distributed by Scripps Howard News Service, www.scrippsnews.com.)