Phytophthora ramorum (Ramorum Blight / Sudden Oak Death)
Phytophthora ramorum
0 host plants · Oomycete
Last updated
Quick Reference
Management
Fall through spring (October-May) during cool, wet periods when leaf wetness persists. Peak infection risk during repeated rain events with daytime temperatures of 50-65°F. Dormant during hot, dry summers (June-August in PNW). In nursery environments with overhead irrigation, year-round risk exists. In southwest Oregon forest settings, peak disease pressure May-October when tanoak is in active growth. # Sources: PNW Plant Disease Management Handbook; OSU Extension
Cool temperatures (50-65°F optimal); persistent leaf wetness from rain, overhead irrigation, or fog; high humidity and dense vegetation; free water required for zoospore dispersal and leaf infection. Sporangia production peaks during cool, wet periods. The pathogen is a cool-weather specialist favored by coastal fog and maritime climates. Rarely establishes in hot, dry regions. # Sources: PNW Handbooks; Wikipedia
Cultural Controls
- Purchase plants from certified nurseries only PNW Plant Disease Management Handbook
- Inspect new plants before purchase; avoid plants with leaf blight, severe leaf loss, shoot dieback, or that have been sitting in standing water WSU HortSense
- Quarantine new plants for 4-6 weeks before adding to the landscape WSU HortSense
- Avoid overhead irrigation; use drip irrigation and space plants for air circulation PNW Plant Disease Management Handbook
- Remove and destroy infected leaves, blighted shoots, and cankered branches promptly; disinfect tools between cuts WSU HortSense
- Do not transport firewood, plant material, or soil from infested areas in California or southwest Oregon PNW Plant Disease Management Handbook
P. ramorum has NOT been found on oak or any Quercus spp. in the PNW. Oregon white oak (Q. garryana) does not appear very susceptible in lab tests. In Washington, most detections have been in nurseries on rhododendrons and camellias, not in wild or cultivated landscapes. The disease is not yet considered widespread in Washington in either cultivated or native settings. P. ramorum infection in the landscape is unlikely unless the plant is a highly susceptible species, was purchased since 2002, and is showing consistent symptoms. The maritime climate (cool, wet springs and falls) is theoretically favorable for this pathogen. The established and expanding outbreak in Curry County, SW Oregon (from 9 sq miles in 2001 to nearly 50% of the county by 2015, with a new Port Orford outbreak in 2021) demonstrates the pathogen's capacity in PNW forest conditions on tanoak.