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 (PDF). See also the
P. ramorum Nursery Chronology and the Acronym Glossary.

History & Background
Since the mid 1990s, P. ramorum has caused substantial mortality in tanoak trees and several oak tree species (coast live oak, California black oak, Shreve oak, and canyon live oak), as well as 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 mid 1990s, and it has since spread to wildland trees in the U.K. and the Netherlands. Although the first P. ramorum-infested California nursery stock was identified in 2001 (Santa Cruz County), the U.S. nursery industry was not widely impacted by the disease until 2003, when the pathogen was detected in California, Oregon, Washington, and British Columbia nurseries.

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/Habitat
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.