Photo of a lone oak at Monte Bello Preserve

Sudden Oak Death

(Gail Bower)

Caused by the water mold pathogen Phytophthora ramorum, sudden oak death (SOD) is killing California coast live oak, black oak, Shreve's oak, canyon live oak and tanoak trees throughout coastal California and Oregon. During the past decade, the pathogen has killed more than one million oak and tanoak trees. 

P. ramorum infections on oaks originally were called “sudden oak death” because of the rapid (2 to 4 week) browning of leaves without an apparent, prolonged period of visible decline. The pathogen spreads with moist winds, accumulation in stream water, soil moisture, and by moving infected vegetation and soil, including material accidentally carried by shoes and tires.

The pathogen also infects other species like California bay trees and coastal redwoods, which often act as carriers, spreading the spores from their leaves and needles. These diseased trees are more vulnerable to other pest organisms like fungi and bark beetles, which can kill them. SOD can cause shifts in plant species composition and declines in populations of animals that rely on oaks, such as black-tailed deer and acorn woodpecker. Dead and dying trees increase the risk of wildfire as they act as more fuel. 

Midpen first detected sudden oak death at Long Ridge Open Space Preserve in 2000. In 2005, Midpen committed $350,000 over 10 years to find ways to prevent and treat SOD by working to identify resistant trees, preventing infection of heritage trees and participating in collaborative research. In 2006, Midpen began working closely with the California Oak Mortality Task Force to achieve these goals. Midpen also partners with scientists on SOD research projects to better understand how to manage the preserves in light of this spreading disease.

To prevent the unintentional spread of sudden oak death, field and construction crews follow “clean practices” when working in infected areas to limit its spread into new areas.

Please note that Midpen may close certain trails during the rainy season to assist in controlling the spread of SOD.

What can I do?

Observe Trail Etiquette:

You can do your part by staying on trails, cleaning your shoes, pet’s paws and tires both before and after your visit to any preserve. Use the boot cleaning bristles that are available at trailheads. 

Learn about the SOD Blitz Survey Project

What is the SOD Blitz Survey Project?

Participate in SOD Blitz events near you.
Natural Resources Protection and Restoration
Natural Resources


California Oak Mortality Taskforce

UC Berkley