Division of Agriculture and Natural Resources
Division of Agriculture and Natural Resources
Division of Agriculture and Natural Resources
University of California
Division of Agriculture and Natural Resources

News Stories

September 10, 2002
Updated May 2006

Frequently asked questions related to Sudden Oak Death


What is sudden oak death?  It's a highly contagious disease that can infect and quickly kill several species of native California oaks.  It first was reported in 1995 in Marin County and has since killed hundreds of thousands of oaks and tanoaks along the state's central coast.

What causes sudden oak death?  It's caused by a fungus-like brown alga called Phytophthora ramorum (phy-TOFF-thoruh ruh-MOR-um), which was identified in 2000 by University of California researchers at the Berkeley and Davis campuses.  P. ramorum is related to the organism that caused the Irish potato famine.  It may have entered the United States on imported nursery plants.

Does this organism affect other trees and plants? Yes, in addition to coast redwood and Douglas fir, the current host list includes: California black oak, coast live oak, Shreve oak, tanoak, rhododendron, California bay laurel, big leaf maple, madrone, manzanita, huckleberry, California honeysuckle, toyon, California buckeye, and California coffeeberry, among many others. P. ramorum also has been found on common nursery plants in Europe and Canada.

What are the symptoms of sudden oak death and related diseases? In oaks, the first symptom is typically a bleeding or oozing of thick, dark reddish-brown sap from the trunk. With tanoaks, the first symptom is dropping of new leaf growth.  Beetles then attack the weakened trees, and in the later stages of decline, decaying fungi are seen on the trunks of oaks and tanoaks. An infected oak will form a girdle, or ring of dead tissue, beneath the bark of its trunk, which blocks the flow of water and nutrients.  A tree may be infected for several years before it dies, although in the final stages of the disease the foliage may turn from green to brown in just a few weeks.

Signs of the disease caused by P. ramorum in redwood have been observed only in the needles and very small branches of redwoods.  In redwoods, the infected sprouts became yellowed and discolored. Douglas fir seedlings showed a similar, but more intense, response.

How do scientists know that these diseases are caused by P. ramorum? First they take living cultures or DNA samples from trees to see if P. ramorum is present.  Then, in the laboratory, they see if they can cause symptoms of the disease by infecting healthy seedlings with that fungus-like organism.  

Where have sudden oak death and other P. ramorum diseases been found? These diseases have been identified in trees and plants in Alameda, Contra Costa, Humboldt, Lake, Marin, Mendocino, Monterey, Napa, San Francisco, San Mateo, Santa Clara, Santa Cruz, Solano and Sonoma counties. Sudden Oak Death also has been identified in Brookings, Oregon.

How are sudden oak death and related diseases spread?  P. ramorum, the organism that causes these diseases, is spread via several types of spores.  During wet weather some affected plants, such as bay laurels, release spores that can travel in moist soil and through the air in rain splash. 

People and animals can also track spores to uninfected areas. These spores also can be carried in the wood, leaves and wood chips from infected trees that are cut down and processed.

What impact will these diseases have on California's landscape?   It is anticipated that sudden oak death will spread to more areas of California.  Since various tree and plant species seem to be more severely affected by P. ramorum than others, it may take years before the full ecological impact of this disease-causing organism will be known.

How can sudden oak death and related diseases be prevented? Preventing the movement of infected leaves, wood and soil is critical to slowing the spread of P. ramorum to other areas.  People who have been in infected areas should clean and disinfect their shoes, and bicycle and vehicle tires with bleach.  Chemical treatments for preventing sudden oak death on oaks and tanoaks are available.

How is movement of tree and plant materials regulated? The California Department of Food and Agriculture, the California Department of Forestry and Fire Protection, and County Agricultural Commissioners, in conjunction with the U.S. Department of Agriculture, are responsible for enforcement of state and federal regulations designed to slow the artificial spread of Phytophthora ramorum. 

Where can I get more information online?

Media contacts:

Webmaster Email: jewarnert@ucanr.edu