Dutch Elm Disease is Devastating for American Elm Trees

Tom Ingram: Ask A Master Gardener

Sunday, August 2, 2020

Q: I have an American Elm that started looking like it had a problem and then died a few weeks later. What in the world happened? JA

A: The culprit was likely Dutch elm disease. I was speaking with Jen Olson of the OSU Plant Disease & Insect Diagnostic Lab recently and she said she was seeing more Dutch elm disease this year than in recent years, which is too bad because it’s one of the most destructive tree diseases in North America.

Dutch elm disease was first discovered in the Netherlands in the early 1900s, but it didn’t take long for it to make its way to the U.S. It arrived around 1930 on beetles who were hitching a ride on some logs headed our way to make furniture.

Quarantine helped control the disease until 1941, but the nation then became more focused on fighting a war. Some estimates suggest there were approximately 77 million elms in North America in the early ’30s. By 1989, more than 75% of those trees were lost.

Dutch elm disease grows in the xylem of the tree. The xylem is the tissue that helps bring water up from the roots throughout the entire tree. You typically start to see evidence of Dutch elm disease in the upper branches with leaves gradually browning, then yellowing and eventually getting dry and brittle.

When adult elm bark beetles emerge from under the bark of infected trees in the spring, they are covered with the Dutch elm disease fungal spores. They look for tender young bark on healthy trees to feed upon. Their feeding leaves wounds that become great places for the Dutch elm disease to take hold.

If you suspect your elm may have Dutch elm disease, you can cut off a branch about 1 inch in diameter from an actively wilting section, peel back the bark and look for discoloration similar to that in the photo (the dark strips). To be sure, you can bring a sample to your local Extension Office, and we will send it off to OSU for positive diagnosis.

If you catch the disease early, the infected areas can be pruned out, but you will need a minimum of 8 to 10 feet of un-infected, streak-free wood below the infected areas.

Fungicides and insecticides are available to help prevent Dutch elm disease; however, these chemical treatments should be applied by a licensed arborist because a fungicide may need to be injected into the tree. You can find a licensed arborist for our area at treesaregood.org.

There are a variety of elm species that are resistant to Dutch elm disease, but none is immune. Resistant varieties include Siberian elm, Chinese or lacebark elm, “Valley Forge,” “Princeton,” “New Harmony” or “American Liberty’.”

Garden tips

• Now is the time to divide and replant crowded hybrid iris (bearded iris). When planting, take care not to plant the rhizomes too deeply. Cover them with an inch of soil or less. Do not mulch iris.
• Water all plants deeply and early in the morning. Most plants need approximately 1 to 2 inches of water per week. Rather than watering daily, water less often and more deeply.
• Some trees, such as sycamores and river birches, lose large numbers of leaves in the heat of summer. Trees do this to reduce water loss from their leaves. It is a coping action by the tree; it is not dying.