The number of trees infected with sudden oak death in California is increasing, and the disease has reached epidemic levels along the coast, researchers say.

The oak-killing pathogen is firmly established in 14 counties - including Santa Clara, Santa Cruz, San Mateo and Alameda - and the number of infected trees in these counties is escalating, University of California researchers reported Friday.

"Sudden oak death is occupying more and more areas within the coastal region," Matteo Garbelotto of the University of California-Berkeley said at a conference of the International Union of Forest Research Organizations in Monterey.

The disease has not significantly expanded its range, however, and does not appear to have spread into the Central Valley, Sierra Nevada or the Pacific Northwest, researchers said.

Dry weather, strict quarantines and other regulatory measures are believed to have hampered the pathogen's expansion into new regions.