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WSU Puyallup Ornamental Plant Pathology

Phytophthora ramorum Symptom ID Guide

Phytophthora ramorum

Plant Pathogen

Phytophthora ramorum is a microscopic organism that has evolved to parasitize plants. It is a particularly harmful plant pathogen because it can kill many of the species it can infect (hosts).


Phytophthora ramorum is a global concern because it can infect many different plant species and spread relatively quickly. Unfortunately, it is challenge to manage because of its broad host range, long-term survival, and the variation in symptoms associated with infections. Furthermore, sometimes management practices (fungicides, biostimulants, etc.) can suppress the expression of symptoms, possibly resulting in the movement of an asymptomatic, but infected, plant.

More Information

More information about Phytophthora species and P. ramorum is available here.

Phytophthora culture Phytophthora sp. sporangia

Sudden Oak Death

Sudden Oak Death

General Information

Sudden Oak Death is a disease caused by Phytophthora ramorum. You can find more information about Sudden Oak Death at the links below:

Symptoms of Disease

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.

Similar Symptoms

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.

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

Phytophthora ramorum Symptoms

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.

Other Online Symptom Guides

USDA Animal and Plant Health Inspection Service (APHIS)

California Oak Mortality Task Force (COMTF)

Forest Research (United Kingdom)

Forest Phytophthoras of the World

Plant Disease Diagnostics

Guidelines for submitting a sample

Visit our Sample Submissions Guidelines webpage for information to help determine whether it will be helpful to submit a plant sample for professional diagnostics.

WSU Plant and Insect Diagnostic Laboratory

The WSU Plant Clinic can provide diagnostic services to identify the cause of symptoms on Rhododendrons and other plant species.

Click here to read their instructions for submitting a sample.

Puyallup Research and Extension Center Sign

Contact Us

Have more questions or concerns?

Contact us for more information or to schedule a site visit and Critical Control Point (CCP) Assessment. Fill out the webform on the right and we will reply shortly after.

Dead Rhododendron shoot
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