University of California-Berkeley researchers said the pathogen was first seen in California forests near a nursery in Santa Cruz and at Mount Tamalpais in Marin County. It eventually spread, killing millions of oak and tanoak trees. The researchers said their findings provide, for the first time, evidence of how the epidemic unfolded in this state.

"In this paper, we actually reconstruct the Sudden Oak Death epidemic," said Adjunct Professor Matteo Garbelotto, who led the study. "We point to where the disease was introduced in the wild and where it spread from those introduction points."

The researchers said the pathogen is currently evolving in California, with mutant genotypes appearing as new areas are infested. That finding suggests movement of infected plants in areas where Sudden Oak Death is established should be minimized, said Garbelotto.

Garbelotto presented the study Wednesday in San Rafael, Calif., during the annual meeting of the California Oak Mortality Task Force. The research that also included Silvia Mascheretti and Peter Croucher will be reported later this month in the journal Molecular Ecology.