WSU Extension

Hortsense

Caption: P. ramorum symptoms on tanoak leaves
Photo by: M. Elliott
  
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Oak : Sudden oak death
(revision date: 7/22/2015)


Biology
Sudden oak death (SOD) is the common name for a disease of oaks (Quercus spp.) and tanoak (Lithocarpus densiflorus) caused by Phytophthora ramorum, a previously unknown and exotic plant pathogen. This pathogen has been found on imported plants in several nurseries in Washington since 2002. Red oaks and tanoak are highly susceptible to SOD, but white oaks are considered resistant. Other common landscape plants that are highly susceptible include rhododendron, kalmia, camellia, viburnum, and Pieris (andromeda). Infected oak and tanoak trees develop stem cankers in the inner bark. These cankers cause the bark to bleed reddish or black droplets of sap. The bleeding is usually not associated with bark cracks or insect holes, although insect holes may be present in infected trees. Bleeding cankers usually occur first in the lower portion of tree trunks, but have been found as high as 60 ft. above the ground. Inside the bark, the cankers appear as discolored blotches, often bordered by dark lines. The cankers expand and girdle the trunk, causing widespread crown death and killing the tree. While IT IS NOT POSSIBLE TO CONCLUSIVELY DIAGNOSE SOD BASED ON VISIBLE SYMPTOMS, the combination of whole crown death accompanied by bleeding of the bark is a good indicator of SOD infection in oak and tanoak. There are many other problems that can cause similar symptoms, including bacterial wetwood, insects, mechanical injury, fungal and bacterial leaf and stem blights, and even other Phytophthora species. The similarity of P. ramorum symptoms to those of other plant diseases and abiotic problems makes field diagnosis very difficult. P. ramorum infection is NOT LIKELY in the landscape UNLESS the plant (1) is a highly susceptible species AND (2) was purchased since 2002 AND (3) is showing symptoms associated with P. ramorum infection (or is located near another plant that meets all these criteria). Currently, the only way to confirm a SOD diagnosis is with laboratory tests.
Management Options

Non-Chemical Management
  • Purchase plants from reputable nurseries only. Carefully inspect all plants before purchase, especially those considered highly susceptible to P. ramorum.
  • Do not buy any plants that have been lying in standing water or that have disease symptoms including leaf blight or lesions, severe leaf loss, or shoot dieback.
  • Quarantine new plants for 4-6 weeks before placing them in the landscape. This disease can be spread by plant debris, contaminated soil, contaminated irrigation water, and even windblown rain, so keep quarantined plants isolated from healthy plants in the landscape and watch closely for any symptoms of disease.
  • Good sanitation is important. Clean up and dispose of fallen diseased leaves and remove blighted and cankered twigs and branches.
  • Avoid overhead watering.
  • If infection with P. ramorum is CONFIRMED by a laboratory test, remove and destroy the infected plant material where practical.
Select non-chemical management options as your first choice!

Chemical Management


None recommended

Images
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Caption: P. ramorum symptoms on tanoak leaves
Photo by: M. Elliott
Caption: Sudden oak death affecting forest trees
Photo by: M. Elliott
Caption: Sudden oak death in a forest
Photo by: M. Elliott