Dutch Elm Disease on American Elm Trees

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.

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 native bark beetle, Hylurgopinus rufipes. The other is a European bark beetle (Scolytus multistriatus), an import that preceded Dutch elm disease itself.

Though both of these beetles carry Dutch elm disease, there is a key difference between the two: native bark beetles only infect elms that are already dying, while European bark beetles infect elms that are healthy. The European bark beetles feed on the cambium just below the thin layer of bark on young twigs up in the canopy of healthy trees. If the beetles have infected only the canopy with Dutch elm disease and the disease has yet to travel down into the rest of the tree, it's possible to prune the canopy and save the larger branches and trunk.

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 and disease is caught early enough, it can be treated by a certified arborist with trunk injections of a fungicide. The diseases progresses rapidly, so you have to act quickly. Pruning will not help you. In fact, pruning branches will deplete the tree of its energy reserves.
• Two rounds of a foliar spray on the canopy before and after bud break will help to prevent a European bark beetle infection, but will not slow down beetle activity if the beetles are already there. The spray is applied to the canopy, as that is where the beetles feed. (An elm can survive a beetle infestation as long as the beetles aren't carrying the fungi.)

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.