Sudden Oak Death (SOD), caused by the water mold Phytophthora ramorum, is killing oak and tanoak trees throughout coastal California and Oregon. The California bay tree has been identified as a main transmitter of SOD, because it hosts the pathogen on its leaves but does not die from it. P. ramorum spores spread from bay leaves to nearby oak trees, which in turn become systemically infected and die.

SOD, first detected on District lands in 2000 on a black oak in Long Ridge Open Space Preserve, has been observed to spread primarily in preserves along Highway 35 between Highways 84 and 9. Infestations occur on approximately half of the District's 26 preserves.

Studies Aim to Protect Canyon Live Oaks

In December 2005, the Midpen Board of Directors adopted a 10-year work plan for addressing SOD, including mapping of potentially resistant trees, treating a select number of heritage trees with a fungicide, and establishing a collaborative research fund for SOD research to help guide land management decisions.

In the summer of 2007, staff noticed dead leaves and bleeding cankers on canyon live oaks in Los Trancos Open Space Preserve. Several canyon live oaks subsequently died, and three trees adjacent to the San Andreas Fault Trail were removed or trimmed. Mortality of mature canyon live oaks from the SOD pathogen, had not been observed elsewhere in California.

In a new study started in November 2009, 12 large oak trees along the Fault and San Franciscan Loop trails in Los Trancos Preserve were sprayed with the fungicide Agri-Fos, considered by the EPA to be safe and nontoxic, and select small-diameter California bay trees near the treated oaks were removed to reduce the risk of infection. Scientists believe Agri-Fos may help protect healthy oaks from Sudden Oak Death. The oaks trees were sprayed again in spring 2010 and then annually each fall.

As part of this same study, bay trees were also removed from around large canyon live oaks along the Ancient Oaks Trail in the Russian Ridge Open Space Preserve. Scientists were concerned that visitors climbing these oaks were contributing to SOD infections due to P. ramorum spores on their shoes being transmitted directly to the tree trunks.

In July 2011, with the help of a talented, young California Conservation Corps (CCC) work crew, the District begin removing approximately 250 bay trees growing within 15 feet of 49 large, healthy oak trees in Los Trancos, Long Ridge, Saratoga Gap, Skyline Ridge and Monte Bello preserves. Research has shown that removing bay trees within 15 feet of oak trees substantially reduces their chances of becoming infected and dying from Sudden Oak Death. By protecting these 49 very large and old oak trees, which represent a legacy in our local forests, this project facilitates the important ecological functions that oak trees on preserve lands in the Santa Cruz Mountains provide: erosion control, water quality, wildlife habitat and scenic, low-impact recreational opportunities.

Sudden Oak Death 10-Year Update

Education and Outreach

The District engages in several activities in an effort to stay current, prevent spread, and educate the public regarding SOD.  District staff are trained by the California Oak Mortality Task Force in detecting and sampling for the SOD pathogen. District staff look for symptomatic plants in new zones, collect samples and send these to the California Department of Food and Agriculture's Plant Pest Diagnostics Center.  All confirmed sites are added to the statewide SOD database and included in the maps available at

In addition, Sonoma State University has been surveying portions of 10 Midpen preserves as part of their statewide Early Detection Study. This survey effort concentrates on identifying zones of infestation rather than individual trees. In 2011, staff and neighbors surveyed preserves and surrounding private land as part of a SOD Blitz to develop more detailed maps of infested areas.

To promote awareness, many preserves are posted with the COMTF "Stop the Spread" SOD signs, and additional information is posted online.

Midpen staff follow field practices for limiting the spread of SOD on preserves, including cleaning of equipment and preventing the relocation of potentially contaminated vegetation and soils.

For more information on best management practices, visit COMTF's Best Management Practices (BMPs) page for a number of wildland and urban-interface area activities and user groups, including recreational users.