Tree Diseases and Pests

Overview

Portland's urban forest is increasingly threatened by invasive pests and diseases that can spread quickly, weaken healthy trees, and create hazardous conditions. The City is working closely with state and local partners to actively monitoring these threats and coordinate response efforts. Our work includes:

• Developing comprehensive management and response plans
• Updating the city tree inventory
• Streamlining permitting for treating or removing affected trees
• Closely monitoring the spread of new pests and diseases. 

How to Help

Protecting Portland's trees depends on early detection and community awareness. Recognizing common signs and learning how to report concerns can help slow the spread of tree diseases and pests.

Dutch Elm Disease

A highly contagious fungus that kills elm trees and can spread to nearby trees, Dutch elm disease continues to endanger Portland's remaining mature elms. Learn more about Dutch elm disease and how to slow its spread.

Emerald Ash Borer

This invasive beetle, recently detected in Oregon, poses an urgent risk, with the potential to kill most ash trees it infests. Learn more about Emerald ash borer and how to slow its spread.

Don't spread tree diseases, keep firewood local!

Keeping firewood local helps stop invasive pests and diseases from spreading to new areas. Buy firewood where you burn it, burn it on site, or choose certified, heat‑treated firewood.

Elm and Ash Tree Pruning and Removal Moratorium

View full size image of Ash and Elm Tree Pruning & Removal Moratorium with graphic of emerald ash borer beetle on tree bark next to a nickel demonstrating its small size.

• Pruning any elm tree is prohibited between April 15 and October 15. Suckers and small branches less than one inch in diameter growing from the base of an elm tree may be pruned year-round.
   
• Pruning and removal of any ash tree is prohibited from April 1 through September 30 when adult beetles are active. Chemical treatment of ash trees is not included in the moratorium and is most effective in the spring and summer after flowering is complete.

Other tree diseases and pests of concern in Portland

• Anthracnose
• Aphids
• Elm Leaf Beetle
• Hawthorne Leaf Blight
• Thousand Cankers Disease
• Verticillium Wilt
• Web Worm and Tent Caterpillar