Dutch elm disease, or DED, is one of the most destructive plant pathogens in the United States and Europe, having killed millions of elms and persisting despite efforts to control it. With approximately 3,500 susceptible elms throughout the city, DED would have a catastrophic impact on Portland’s urban forest if allowed to prevail unchecked. PP&R Urban Forestry works diligently to manage the impact of DED in Portland.
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 transporting infected elm wood, also spreads the disease.
Infection can be prevented with commercial fungicides. There are two fungicides currently on the market, Arbotect (thiabendazole hypophosphite) and Alamo (propiconazole). The two fungicides vary in their application systems and their price ranges, but both 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 retard the spread of DED in affected urban environments.
On June 10, 1987, Portland City Council passed Ordinance No. 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 managing 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 fungus is detected, steps are taken to remove the tree.
- Removal: Swift removal lessons 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 are 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 is 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.
Pruning any species or varieties of elm trees between April 15 and October 15 is prohibited within the City of Portland. 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. 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.