By Don MacGillivray
With approximately 3,000 potentially susceptible elms throughout the city, Dutch Elm disease (DED) would have a catastrophic impact on Portland’s urban forest if it were allowed to prevail unchecked. 
A total of 1,048 elm trees have been removed in Portland due to DED since it first arrived about 25 years ago.
Forty-two trees were removed due to Dutch elm disease in 2012. Nineteen of these trees were on Parks-owned or managed property, thirteen were in the street right-of-way, and ten were on private property.
The majority of elms removed were in SE Portland in the neighborhoods of Hosford-Abernethy, Eastmoreland, and Sellwood-Moreland. Ladd’s Addition is the obvious example of the challenge.
Ladd and Eliot avenues are lined by about a mile and half of elm trees and are an important feature of this historic neighborhood, but each year, Portland loses about thirty-five elm trees or 1.2 percent of the city’s elm tree inventory.
This is a very small loss rate when compared with the east coast, where 30 to 50 percent of a town’s elm population could die within a year.
Dutch elm disease is one of the most destructive plant pathogens in the United States and Europe, having killed millions of elm trees despite persistent efforts to control it. Caused by two fungi which invade the tissue of elms and prohibit water movement in the tree, an infected tree can die within weeks of showing the first symptoms.
First identified in Europe in 1922, it is believed that DED originated in Asia, since Asian elms are resistant to the fungus, but the elms in America and Europe are very susceptible to the disease.
By 1972, the European bark beetle, the primary DED vector, was found in all but four states. A single case was discovered in Portland’s Overlook Park neighborhood in 1977. The second case was discovered ten years later in Laurelhurst.
Since then Portland’s Urban Forestry’s Elm Monitoring program has worked to slow the spread of DED throughout the city.
Bark beetles breed, feed, and overwinter in elm wood. The fungus they carry is rapidly spread through root grafts, that form between trees growing in close proximity like in Ladd’s Addition. Parks Urban Forestry department removes infected elms in the right-of-way planting strips, but on private property, the owner is required to remove the tree at their expense within 15 days of diagnosis.
Portland Parks Urban Forestry department inoculates some elms on a 3-year rotation, targeting significant elms in Portland’s parks and public spaces. The nonprofit Save Our Elms (SOE) and their affiliates raise money and organize neighborhood elm inoculations also on a 3-year cycle.
For many, it is a traumatic experience to lose an elm and have it cut down leaving a hole in an otherwise ideal landscape. SOE has worked for 15 years to retard the loss of the elm trees. They work to inoculate American Elm trees to resist Dutch Elm Disease.
Affiliate groups operate in Ladd’s Addition, Mount Tabor, Richmond, Laurelhurst, and Eastmoreland neighborhoods. Each affiliate conducts local fundraising activities to purchase fungicide to inject into the root flare of elm trees.
SOE works with the Friends of Trees to replace trees with disease-resistant varieties of hybrid elm trees. It is hoped that Portland’s elm population will remain stable over time.
There is no known cure for DED, but aggressive pruning of infected branches coupled with inoculation will delay mortality. In many cases, fungicides are the only effective treatment, but are not 100 percent effective. Bark beetles are active during spring and summer months and are attracted to open wounds left by pruning.
Portland observes a moratorium on pruning elms between April 15 and October 15.
Wilting or dropping leaves, dead branches or patches of yellow or brown leaves, called “flagging,” during spring and summer when trees should be green is the indication of Dutch elm disease.
Save Our Elms is a membership organization that helps to preserve the unparalleled urban canopy provided by the elm trees in Portland and is supported by memberships, volunteers, and events to do their work.
If you are interested in assisting their efforts, consider joining. Information is on their website or through the City of Portland’s Nature Urban Forestry department at 503.823.8178. Elm trees are a community resource that captures storm-water runoff, cleans the air, muffles traffic noise, cools the neighborhood in summer, and adds to the positive ambiance of the neighborhood.