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Washington State University
WSU Puyallup Ornamental Plant Pathology

Information for Master Gardeners

Sudden Oak Death

Sudden Oak Death is a disease caused by Phytophthora ramorum, an exotic microscopic organism that was accidentally introduced into the western US. The disease has killed thousands of trees in California and Oregon, and threatens natural environments in Washington.

Phytophthora ramorum also has a broad host range and presents a unique challenge to many industries in Washington. The effects on ornamental plant species are of particular concern. Therefore,  we have prepared several resources to help you identify and report possible occurrences, included below.

Please contact us if you additional questions or concerns.

Sudden Oak Death Presentation Link
Click on the image above to download a PDF presentation about Sudden Oak Death

Symptoms of Disease

There are many diseases and pests that produce symptoms similar to those of Sudden Oak Death. This guide will help you to rule out SOD by identifying some of the more commonly encountered pest problems. In addition, it will help determine whether the plant is potentially infected with P. ramorum and the need for a sample to be collected.

Currently there are 5 tree species native to western North America that can be killed when they are infected with Phytophthora ramorum. These are tanoak (Lithocarpus densiflorus) and four oak species – Coast live oak (Quercus agrifolia), Canyon live oak (Q. chrysolepis), California black oak (Q. kelloggii), and Shreve oak (Q. parvula var. shrevei).

In Europe, P. ramorum has been found to cause stem cankers leading to death of European beech (Fagus sylvatica), European sycamore (Acer pseudoplatanus), Horse chestnut (Aesculus hippocastanum), Northern red oak (Q. rubra), Sessile oak (Q. petraea), Southern beech (Nothofagus obliqua), Southern red oak (Q. falcata), and Turkey oak (Q. cerris). None of these tree species are native to Washington State, although they can be found planted as ornamentals.

Some tree diseases that produce symptoms similar to P. ramorum are the following:

Phytophthora ramorum produces leaf blotches, twig cankers, and shoot die-back on many plants. In some cases the shoot dieback may be severe enough to kill the plants (e.g., huckleberry and madrone), but in general they do not usually die as a result of P. ramorum infection. These are the most common hosts in Washington State.

Additional Resources


YouTube Videos about Phytophthora