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Tree Diseases and Pests

A photo of tree leaves with Anthracnose disease.
These tree pests and diseases are common in the Pacific Northwest, but this is not an exhaustive list.
On this page

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.

  • Susceptible species: Pacific and eastern dogwoods (Cornus nuttallii and Cornus florida), London planetree, American sycamore, and some other common hardwoods (species of ash, oak, maple, and walnut).
  • Treatment: Prompt removal of affected leaves can help reduce next year’s outbreak. Anthracnose rarely kills trees. If removal is necessary, replace with a tree species that is resistant to anthracnose.


Trees affected by aphids are commonly misreported as dripping sap. These small insects suck sap from leaves and excrete a clear, sticky honeydew that can drop onto cars and anything under the tree. Honeydew sometimes encourages a black fungal growth called sooty mold.

  • Susceptible species: Many species, but lindens and tulip poplars are particularly susceptible.
  • Treatment: Leaves of small trees can be sprayed with water to mechanically remove aphids.

Dutch Elm Disease

Dutch elm disease is caused by a fungus and is highly lethal to American and European elms. 

Blockage of water-conducting tissues indicated by flagging (localized leaf wilt, yellowing, and browning) results from infection with the lethal fungus. The disease is spread primarily by elm bark beetles. Because fresh pruning wounds attract the elm bark beetle, elm pruning is restricted to times of beetle inactivity (October 15 to April 15).

  • Susceptible species: Most elm trees.
  • Treatment: DED has no known cure. Prompt removal and destruction of diseased trees at approved facilities limits disease spread, and dead wood pruning reduces beetle habitat. Replacing removed elms with a tree species other than elm is recommended.

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 (EAB) are metallic green beetles that feed on ash trees. The first observation of EAB on the west coast occurred in Forest Grove June 2022.

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

Verticillium wilt is caused by a soil-dwelling fungus that causes leaves to brown and die. 

 Verticillium wilt usually only appears in damaged or otherwise stressed trees.

  • Susceptible species: Certain species of maple, ash, redbud, dogwood and linden trees to various degrees.
  • Treatment: Plant verticillium-resistant species and reduce stress on affected trees.

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

  • Susceptible species: Many species, including alder, crabapples, and madrones.
  • Treatment: Mechanical control by pruning out infected areas can be effective.


More detailed images of common tree deceases.