Main Street USA in the first half of the 20th century was typically lined with American elm trees. These majestic giants, which unfurled their weeping branches Rapunzel-like over the heads of passersby, afforded shade on hot summer afternoons.
But Dutch elm disease (Ceratocystis ulmi) changed all that with its quick spread. There's been some positive movement in eradicating this tree disease, but here's what you need to know if you are lucky enough to still have American elms on your property.
What Is Dutch Elm Disease?
Dutch elm disease is named so because it was first identified in the Netherlands in 1921. It quickly spread to Europe, North America, and parts of Asia, where elms succumbed to the fungus. It's considered a vascular wilt disease or wilt fungus that grows in the sapwood of elms. The fungus grows and reproduces, and it's carried in the water that flows through the tree's branches and stems. The flow, however, stops as the fungus grows and blocks water movement, causing severe wilting.
History of Dutch Elm Disease and the American Elm
American elm trees (Ulmus americana) are the most susceptible of all to Dutch elm disease. American elm trees are also known as water elms, soft elms, white elms, or Florida elms. They are found throughout eastern and central North America, and their range extends as far south as northern Texas and Florida.
Cleveland, Ohio, witnessed the first case of Dutch elm disease in the U.S. in 1930. This silent killer arrived in a shipment of infected logs from France. Dutch elm disease spread east quickly; within two years, American elm trees in New Jersey were falling prey to the deadly fungus.
How Elm Trees Are Affected
For all the tranquility such mass plantings bestowed, this monocultural practice was one of the culprits in the downfall of American elm trees. The deadly fungus, it turns out, can spread underground from the roots of one victim to the roots of another nearby. This is what happened when the roots of adjacent American elm trees "grafted" together, essentially linking the lives of what had been two distinct entities.
The demise of one would become the demise of the other. The monoculture and its consequent root grafting meant that an infected sap could pass from one American elm tree to another in a chain reaction that would decimate a whole row along a street. Soon, whole streets were filled with infected trees with yellowed, wilting leaves dangling from branches and peeled twigs discolored brown because sap and water flow ceased.
The planting of American elm trees en masse was not the sole culprit, though. The microscopic spores of the fungus are also transmitted from diseased victims to healthy specimens by two kinds of beetles that tunnel under the bark. One is a European bark beetle (Scolytus multistriatus), an import that preceded Dutch elm disease itself. The other is a native bark beetle, Hylurgopinus rufipes. Photos of both carriers of Dutch elm disease can be found at the Utah State extension site, as well as additional information about Dutch elm disease.
The beetles, also aptly called boring beetles, burrow under the bark of the elm trees. There, they tunnel into the tree and lay their eggs. The problem lies in the fungal spores that the beetles carry on their bodies and deposit within the trees.
Thanks to the plant cloning work of tree geneticist Alden Townsend, American elm tree clones resistant to Dutch elm disease became a reality and the prognosis for Ulmus americana is now good. In the late 1990s, after 25 years of work with U. americana, Townsend announced he'd succeeded with two new strains, U. americana 'Valley Forge' and U. americana 'New Harmony.' Townsend's clones are now on the market.
Dutch Elm Disease Guidelines
If you can't find or plant clones, or you are trying to save a long-established elm tree from the disease, follow these guidelines:
- Prune dead or dying branches off American elms from fall to late winter. This procedure, called limbing, is best handled by professionals.
- Avoid pruning American elms from April through August. The elm bark beetle is attracted to freshly cut elm and is most active during this period.
- Be on the lookout for the signs of Dutch elm disease. Leaves of infected American elms will wilt in the summer. They will first turn yellow, then curl, and finally become brown. The signs usually first appear in the crowns of American elms.
- If signs appear, dispose of infected American elms quickly and properly to save surrounding trees. In rural areas, they may be burned. In urban areas, take them to a designated disposal site.
- There are chemical control methods to slow down beetle activity. Trunks can be sprayed with appropriate insecticide, and foliage can be sprayed to kill feeding adults. Other therapeutic control of Dutch elm disease should be handled by a professional arborist.
It may be tough to identify Dutch elm disease on a tree. The signs often mimic a tree experiencing water stress or other common disorders. It may help to talk to your town's tree warden or your county's extension office for positive identification of the disease.
The plant cloning work of tree geneticists continues in hopes of developing new American elms that will be even more resistant to Dutch elm disease. If you want to plant American elm trees on your property to enjoy the shade of their enormous outstretched arms and full canopies, consider buying the disease-resistant varieties. You can also try Asian elms because they are more resistant to Dutch elm disease than American elms. Also consider planting them in containers as a safety measure so that the roots can't touch each other.
Note that American elms are considered one of the worst trees to plant for allergy sufferers. For those who are not allergy sufferers, American elms make for splendid specimen plants. American elms are cold hardy to zone 3, and they can grow up to 120 feet high.