Common Diseases: Sudden oak death

Biology

Sudden Oak Death (SOD) is the common name for a disease caused by Phytophthora ramorum, a previously unknown and exotic plant pathogen. In California, it has killed hundreds of thousands of oak and tanoak trees. P. ramorum can infect a large number of plant species including Douglas-fir, grand fir, rhododendron, viburnum, big leaf maple, vine maple, madrone, Pacific yew, salal, larch, and many others in Washington’s natural and urban landscapes. The pathogen can cause severe symptoms and death in tanoak and some oak species, but on many hosts the organism causes relatively minor symptoms (usually foliar or stem blights). The similarity of P. ramorum symptoms to those of other plant diseases and abiotic problems makes field diagnosis difficult. IT IS NOT POSSIBLE TO CONCLUSIVELY DIAGNOSE THIS PROBLEM BASED ON VISIBLE SYMPTOMS ONLY. P. ramorum can spread through forest landscapes by wind and wind-driven rain. The pathogen survives in infested plant material, litter, soil, and contaminated water and can be moved long distances in nursery stock. Western Washington is a “high risk” area for diseases caused by P. ramorum because of favorable environmental conditions and the abundance of susceptible host plants. In Washington, SOD (also known as Ramorum leaf blight) has so far been found only in association with infected nursery plants and water runoff from infected nursery sites. While this disease has the potential to become a serious and economically significant problem, it is not yet considered to be established in Washington in either cultivated or native landscapes. Red oaks and tanoak are highly susceptible to SOD, but white oaks are considered resistant. Other plants that are considered highly susceptible include rhododendron, camellia, viburnum, kalmia, and Pieris (andromeda). For more information, see Ramorum leaf and shoot blight pages for these hosts. 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

Use Integrated Pest Management (IPM) for successful plant problem management.

Non-chemical Management

Select non-chemical management options as your first choice!

• 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 adding them to the landscape. This disease can be spread by plant debris, contaminated soil, and contaminated irrigation water and run-off, 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.

Chemical Management

IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides.

None recommended.

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