Understanding and Managing Sudden Oak Death in California

1.1.2. Origin and Distribution of Phytophthora ramorum in California

Sudden oak death was first widely recognized in California in 1995, when tanoaks began dying in areas of Marin, Santa Cruz, and Monterey Counties (Svhira 2001). It was not until 2000 that P. ramorum was diagnosed as the cause of SOD (Rizzo and others 2002).

The pathogen was introduced into California on contaminated nursery stock or other plant materials, possibly in the 1980s. Genetic analyses and other data suggest that P. ramorum moved via infested nursery stock to multiple locations in several counties, including Santa Cruz, Marin, and Sonoma, where it spread from nursery-grown plants into the adjacent forests. Eventually, the larger forest infestations served as sources for continued spread of the pathogen to other forests (Croucher and others 2013).

After its accidental introduction, P. ramorum became established in native forests with susceptible host plants and favorable conditions for disease development. P. ramorum is now found in coastal California forests from Humboldt to Monterey Counties, and in Curry County in southern Oregon. It is also found in the counties bordering the San Francisco Bay. See www.oakmapper.org for the most recent distribution maps. All counties with confirmed P. ramorum in natural settings (table 1-4) are under State and Federal quarantine. Quarantined counties are subject to regulations regarding the movement and use of susceptible plants. County Agricultural commissioners enforce both California and federal regulations related to this pathogen.

County, state, and federal agencies in cooperation with nursery managers have made considerable efforts to eliminate P. ramorum from nursery plants. These efforts have greatly reduced the risk of spread of the pathogen in nursery plants. Nonetheless, contaminated nursery stock remains a potential means of pathogen spread within California as well as between states and countries.

The range of P. ramorum in California (fig. 1-1) continues to expand. Within infested counties P. ramorum is not uniformly distributed. Stands of uninfected trees commonly occur in counties where SOD is common. The distribution of SOD-affected trees is typically patchy, even within infested stands.

Table 1-4—California counties with confirmed Phytophthora ramorum and under state and federal quarantine (February 2012)

Alameda Contra Costa
Humboldt Lake
Marin Mendocino
Monterey Napa
San Francisco San Mateo
Santa Clara Santa Cruz
Solano Sonoma

  Range of P ramorum in CA

Figure 1-1—Approximate distribution of Phytophthora ramorum in California as of July 2012. Note Oregon location, just north of the California border. Upright magenta triangles are positive plant and water samples from SODMAP (University of California-Berkeley 2012). Inverted violet triangles are officially confirmed records from OakMapper (Kelly and others 2012).