About the Pathogen

Photo: Jennifer Parke, OSUPhytophthora species are water molds that are well known plant pathogens. They are water-loving and produce plentiful spores in moist, humid conditions. While most foliar hosts do not die from the disease, they do play a key role in the spread of P. ramorum, acting as breeding grounds for inoculum, which may then be spread through wind-driven rain, water, plant material, or human activity. Trunk hosts such as oaks are considered terminal hosts – the pathogen does not readily spread from intact bark cankers – and they become infected only when exposed to spores produced on the leaves of neighboring plants.

The APS Education Center also has a disease lesson for P. ramorum which includes illustrations of proposed disease cycles, among other biological information. For more specific details about P. ramorum, go to the Global Invasive Species Web Database. Visit Forest Phytophthoras of the World for science-based information to aid in the understanding and management of the world’s forest Phytophthora species.

The movie below shows a sporangia releasing zoospores.

Pathogen Distribution

In California, infestations in natural settings have been found in 15 central and northern coastal counties (map below): Humboldt, Trinity, Mendocino, Lake, Sonoma, Napa, Solano, Marin, Contra Costa, Alameda, San Francisco, San Mateo, Santa Clara, Santa Cruz, and Monterey.

In addition, P. ramorum has been found in Curry County, Oregon. In Europe, P. ramorum has been identified on nursery plants in Germany, Spain, France, Poland, Belgium, Sweden, Italy, the Netherlands, and the U.K. It has also been isolated from forest trees in the U.K. and in the Netherlands.

It is not considered native to North America or Europe, but its native range is currently unknown.

For more maps, please see the Maps page.