Dutch Elm Disease

No elm pruning from April 15 to October 15

To help protect Portland's elm trees, pruning any species of elm between April 15 and October 15 is prohibited.

The City Forester, or the Forester's representative, may waive this prohibition when it is necessary to remove hazards or maintain clearance. Suckers and small branches less than one inch in diameter growing from the base of an elm tree may be pruned year-round.

To report a potential case of Dutch elm disease, please contact the Elm Monitor at pkelm@portlandoregon.gov. See below for more information on identifying Dutch elm disease.

For more information on elm care and management, please see Urban Forestry's Elm Management Strategy. For tree permitting questions or concerns, email trees@portlandoregon.gov or call (503) 823-TREE (8733).

Elm Wood Quarantine

All pruned elm wood must be disposed of properly to prevent the spread of infection. Elm wood is not allowed to be stored as firewood. All elm wood must either be chipped or debarked and buried. For more details, see the procedures for elm wood disposal.

Visit the Oregon Department of Agriculture website for the State of Oregon elm quarantine restrictions.

Dutch Elm Disease Signs and Symptoms

View full size image of Elm flagging in the canopy due to Dutch Elm Disease.

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

The main visual symptom of Dutch elm disease, known as "flagging," is a sudden wilting or drooping of leaves in the tree, often on a single branch or limb. Flagging leaves quickly turn from grey green to brown as the fungus invades the vascular tissue of the tree, blocking the tree's water supply. The leaves often remain wilted before eventually dropping as the flagging spreads throughout the canopy.

The Dutch elm disease fungus can spread through bark beetles, root grafts, and human activity. Elm bark beetles breed, feed, and overwinter in elm wood. In infected trees, spores of the Dutch elm disease fungus stick to the backs of bark beetles and are transported to healthy elms when beetles emerge to feed.

The fungus spreads most rapidly through root grafts, which form between trees growing in close proximity, such as within a planting strip. Human activity, such as pruning and transporting infected elm wood, also spreads the disease.

Treatment

View full size image of Urban Forestry staff performing Arbotect inoculations in the North Park Blocks.

Fungicide can be injected into elm trees as a preventative treatment. A Chemical Treatment Permit is required, and a certified arborist must supervise the fungicide injection procedure.

Two fungicides currently on the market to prevent Dutch elm disease infection are Arbotect (thiabendazole hypophosphite) and Alamo (propiconazole). These two fungicides vary in their application systems and price ranges, but both may help prevent infection in elms by disabling fungal spores. Please note, this means that they only work to limit infection via elm bark beetles, and do not prevent infection via root grafting from adjacent trees. These fungicides are not 100% effective, and there are no known cures for Dutch elm disease. However, it is possible to slow the spread of Dutch elm disease.

If you would like to bring your community together to fundraise, inoculate elm trees, and replant trees that have been removed due to Dutch elm disease, you can contact your neighborhood association for help.

Urban Forestry’s Elm Management Strategy

The City of Portland has been actively managing Dutch elm disease for decades and has developed a five-prong management approach:

1. Monitoring: Each summer, as the symptoms become apparent, Urban Forestry monitors the city's elm populations. When flagging is observed in an elm, Urban Forestry staff sample the flagging branches and look for streaking in the sapwood. Healthy elm wood is uniformly blonde. Dutch elm disease infection causes brown or grey linear discolorations parallel to the twig. If this streaking is present in the branches, the sample is sent to the Oregon State University Plant Pathology Clinic to try to cultivate the fungus. If the presence of the fungus is detected or the symptomatic elm continues to decline due to likely Dutch elm disease infection, steps are taken to remove the tree.
    

2. View full size image of Urban Forestry arborist crew collecting an elm sample for DED testing.

   Removal: Swift removal reduces the opportunity for the infected tree to attract bark beetles or for the fungus to spread to adjacent trees via root grafts. All trees found to have Dutch elm disease must be removed as quickly as possible. After removal, stumps are ground promptly to prevent infection via root grafts. As part of the City's new Street Tree Care program, starting May 1, 2026, Urban Forestry will begin removing diseased elm trees in public rights-of-way at no cost to property owners. Whenever possible, we will replant with a diversity of trees that are more resilient to pests and diseases. This work is funded by the Portland Clean Energy Community Benefits Fund.
    

3. Sanitation: All elm wood must be disposed of in a controlled manner either by chipping or de-barking and burying so as not to attract bark beetles to the infected wood. All tools used on elm trees are disinfected before and after each use so they do not become contaminated with fungal spores. Portland also observes a moratorium on pruning elms between April 15^th and October 15^th annually. Bark beetles are active during the spring and summer months and are attracted to the open wound sites left by pruning. Deadwood pruning should be conducted during the winter months to further reduce sites that are attractive to bark beetles.
    
4. Inoculation: Urban Forestry inoculates approximately 260 elms on a three-year rotation with the fungicide Arbotect (thiabendazole hypophosphite), targeting mature elms in Portland's parks and public spaces.
    
5. Education: One of the best strategies Urban Forestry has for protecting Portland's elm trees is education. Raising community awareness about the epidemic facilitates swift identification of the disease and eases the loss felt when infected trees are removed. Informed communities also support the management of Dutch elm disease by ensuring appropriate tree care and maintenance, as well as raising funds and volunteer support to inoculate neighborhood elm trees and replant removed elms.

Frequently Asked Questions

DED Annual Reports

• 2013 – Current DED Annual Reports (Links to City Archives & Records Management Efile system)

Additional information

• How to Identify and Manage Dutch Elm Disease (U.S. Forest Service)
• Dutch Elm Disease (American Phytopathological Society)
• Dutch Elm Disease (The Morton Arboretum)