Dutch elm disease, or DED, is one of the most destructive plant pathogens in the United States and Europe. It has killed millions of elms despite efforts to control it. With approximately 3,500 susceptible elms throughout Portland, PP&R Urban Forestry works diligently to manage the impact of DED.
No Elm Pruning From April 15 to October 15
To help protect Portland's elm trees, pruning any species of elm between April 15 to October 15 is prohibited.
The City Forester, or the Forester’s representative, may waive this prohibition when it is deemed necessary to remove hazards or maintain clearance. Suckers and small branches less than 1-inch in diameter growing from the base of an elm tree may be pruned year-round.
Thank you for helping to protect Portland's elms. For more information, read Urban Forestry's Elm Management Strategy.
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 de-barked and buried. For more details, see the procedures for elm wood disposal.
Visit the Oregon Department of Agriculture website for State of Oregon elm quarantine restrictions.
Dutch Elm Disease Description
Dutch elm disease (DED), is caused by a fungus and is highly lethal to American and European elms. The fungus spreads through:
- Root grafts
- Elm bark beetles
- Human activity such as pruning, moving, and storing elm wood
The main visual symptom of DED, 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.
Because fresh pruning wounds attract the elm bark beetle, elm pruning is restricted to times when the beetle is not active. Also, the state of Oregon has declared an emergency quarantine of all elm wood. Elm wood must be chipped or de-barked and buried, and cannot be stored for firewood.
Most Susceptible Species
- American elm (Ulmus americana)
- Dutch elm (Ulmus x hollandica)
- English elm (Ulmus procera)
- Wych elm (Ulmus glabra)
- Camperdown elm (Ulmus glabra 'Camperdownii')
- Smoothleaf elm (Ulmus minor)
Recent elm hybrids have been developed to be less susceptible to DED. This does not necessarily mean that they are immune to the disease, and proper precautions should be taken with their care and maintenance.
Fungicide can be injected into elm trees as a preventative treatment. A certified arborist must supervise the fungicide injection procedure. Save Our Elms and affiliates, as well as local nonprofit organizations bring communities together to fundraise, inoculate elm trees, and replant trees that have been removed due to Dutch elm disease.
Two fungicides currently on the market to prevent DED infection are Arbotect (thiabendazole hypophosphite) and Alamo (propiconazole). These two fungicides vary in their application systems and their price ranges, but both may help prevent DED infection in elms by disabling fungal spores.
These fungicides are not 100% effective and there are no known cures for DED. However, a dedicated community with clear goals can take steps to slow the spread of DED in affected urban environments.
Biology of DED
DED is caused by a fungus, Ophiostoma spp., which invades the vascular tissue of elms and prohibits water movement in the tree. There are three ways the DED fungus spreads: bark beetles, root grafts, and human activity.
Elm bark beetles breed, feed, and overwinter in elm wood. In infected trees, spores of the DED 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.
Human activity, such as pruning and transporting infected elm wood, also spreads the disease.
Urban Forestry’s Elm Management Strategy
On June 10, 1987, Portland City Council passed Ordinance 159750, declaring Dutch elm disease-infected trees a nuisance and instating an emergency.
The ordinance specifies that it is unlawful for elm trees infected with DED to remain on any lot or parcel of land in the city. This ordinance was codified in Title 11 Trees section 11.60.060C by Portland City Council in 2011.
Using a five-pronged management approach, Urban Forestry addresses DED from the angles of preventing the disease and regulating the elms that are already infected.
- Monitoring: Each summer, as the symptoms of DED become apparent, Urban Forestry monitors the city's elm populations. The main visual symptom of DED, 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. When flagging is noticed in an elm, Urban Forestry crews sample the flagging branches and look for streaking in the sapwood. Healthy elm wood is uniformly blonde; DED 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 fungusis detected, steps are taken to remove the tree.
- 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 to prevent infection via root grafts.
- Sanitation: All elm wood must be disposed of in a controlled manner 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 use so they do not become contaminated with fungal spores. Portland also observes a moratorium on pruning elms between April 15th and October 15th annually. Bark beetles are active during the spring and summer months and are attracted to the open wound sites left by pruning. Deadwood pruning conducted during the winter months to further reduce sites that are attractive to bark beetles.
- Inoculation: Urban Forestry inoculates approximately 450 elms on a 3-year rotation with the fungicide Arbotect (thiabendazole hypophosphite), targeting significant elms in Portland’s parks and public spaces. The nonprofit organization Save Our Elms and their affiliates raise money and organize neighborhood elm inoculations. Save Our Elms inoculates about 300 elms each year with the fungicide Alamo (propiconazole).
- 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 raising funds and volunteer support to inoculate neighborhood elm trees and replant when elms are removed.