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.

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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.

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