Sudden Oak Death Overview

Multimedia overviews are also available: video overview by Dr. David Rizzo, UC Davis, on UCCE Sonoma website and radio interview with Dave Rizzo, Yana Valachovic, and Leonel Arguello.

Phytophthora ramorum is the cause of both Sudden Oak Death, a forest disease that has resulted in widespread dieback of several tree species in California and Oregon forests, and Ramorum blight, which affects the leaves and twigs of numerous other plants in forests and nurseries. For the complete disease chronology: P. Ramorum Chronology.  See also the P. ramorum Nursery Chronology and the Acronym Glossary.

History & BackgroundDead oaks, Marin County. Photo: Kent Julin, Marin County

Since the mid-1990s, Phytophthora ramorum has killed millions of tanoak trees and several oak tree species (coast live oak, California black oak, Shreve oak, and canyon live oak), and caused  twig and foliar diseases in numerous other plant species, including California bay laurel, Douglas-fir, and coast redwood. The pathogen was also discovered in European nurseries in the mid1990s, and it has since spread to gardens and wildland trees most notably in the U.K. where Japanese larch in plantations are being killed in large numbers. Although the first P. ramorum-infested rhododendron nursery plants, were identified in 2001 (in Santa Cruz County, CA), the U.S. nursery industry was not widely impacted by the pathogen until 2004, when a few large West Coast nurseries inadvertently shipped over  a million potentially infected rhodendrons and camellia plants throughout much of the United States with detections in 176 nurseries in 21 states. The origin of Phytophthora ramorum is not known.

Ecological Threats

Possible threats include a change in species composition in infested forests and therefore, in ecosystem functioning; loss of food sources for wildlife; a change in fire frequency or intensity; and decreased water quality due to an increase in exposed soil surfaces.

Environment/HabitatDead Tanoaks, Big Sur. Photo: Dave Rizzo, UC Davis

P. ramorum thrives in cool, wet climates. In California, coastal evergreen forests and tanoak/redwood forests within the fog belt are the primary habitat. Research in California forests has shown that the greatest predictor of P. ramorum is the presence of California bay laurel (Umbellularia californica). Nurseries outside of these cool, moist areas often create microclimates which mimic the preferred environment of P. ramorum and allow it to grow and spread far from the coast.