These tree pests and diseases are common in the Pacific Northwest, but this is not an exhaustive list.
If you need additional assistance diagnosing your trees, contact the Oregon State University Plant Disease Clinic or your local arborist.
Anthracnose
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
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