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Tree Diseases and Pests

These tree pests and diseases are common in the Pacific Northwest, but this is not an exhaustive list.

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If you need additional assistance diagnosing your trees, contact the Oregon State University Plant Disease Clinic or your local arborist.


Anthracnose is a fungal disease that causes leaves to brown and drop in middle to late summer. It is common on Pacific and flowering dogwoods, London planetrees, American sycamores, and species of ash, oak, maple, and walnut


Aphids are insects that feed on plant sap and produce a sticky substance called honeydew. Aphids are most found on linden and tulip trees

Dutch Elm Disease

Dutch elm disease is caused by a fungus and is highly lethal to American and European elms. The symptom of Dutch elm disease is flagging, a wilting and browning of leaves, usually on a single branch or portion of the canopy

Elm Protection Program and Dutch Elm Disease (DED) page

Elm Leaf Beetle

Elm leaf beetles feed on elm leaves, leaving lacy holes in the leaves and eventually causing leaf drop. Most elm tree species are susceptible

Emerald Ash Borer

Emerald ash borers are metallic green beetles that feed on ash trees. Emerald ash borer has not been found in Oregon or the west coast yet

Hawthorne Leaf Blight

Hawthorne leaf blight manifests as small reddish-brown spots; the area between spots will yellow and the leaves will eventually fall off. Several species of Hawthorne are susceptible to leaf blight

Thousand Cankers Disease

Thousand cankers disease is a fungus that causes cankers to develop under the bark of an infected tree, often leading to the death of the tree. Only walnut trees are susceptible to thousand cankers disease

Verticillium Wilt

Verticilium wilt is caused by a soil-dwelling fungus that causes leaves to brown and die. Maple, ash, dogwood and linden trees are susceptible to verticillium wilt

Web Worm and Tent Caterpillar

Several species of caterpillar create a web or tent in the branches of trees and feed on the tree’s foliage. Web worms or tent caterpillars are commonly found on fruit trees, cottonwoods, aspen, ash trees, among other species