American Elms, Ulmus americana, in fall

Disease Resistant American Elm Trees

American Elms, Ulmus americana, are loved for their graceful, stately shape, with branches like spreading fountains, and their green leaves that turn gold in fall. The American elm once graced city streets, town squares, and farmsteads throughout eastern North America.

The American Elm is the most graceful of form and potentially attains the largest size of all native elms. The form and stateliness of the tree has been appreciated by observers from colonial times on, including Oliver Wendell Holmes and Henry David Thoreau. Probably every eastern town in America had its elms, as did many tree-lined boulevards in our larger cities.

After Dutch Elm Disease wiped out the American Elm across North America, Elm cultivars have been identified that provide Dutch Elm Disease resistance.

Over the last 100 years there have been dozens of American elm selections. Unfortunately, most did not survive the ravages of Dutch Elm Disease and have been lost and forgotten. All over the Eastern and Northern U.S., people witnessed the rapid loss of mature American elms. The disease is particularly virulent in the Northeast, where the solitary isolated mature elm has become a rare site.

Disease Resistant Elm Cultivars

American Elm, Ulmus americana, leaf

  Princeton:  Selected in 1922. Vigorous growth rate with very upright form. Available in most garden centers and also through mail-order.
  Prairie Expedition:  A 2004 North Dakota State University selection. Classic vase-shaped American elm with outstanding autumn gold color. Winter hardy to USDA zone 3.
  New Harmony:  A USDA selection that appears to have superior form when compared to Princeton and Valley Forge.
  St. Croix:  Selected by Mark Stennes from a massive parent tree in Afton, MN, this elm joins the ranks of Dutch elm disease-resistant elms with a Minnesota twist.

Elm Maintenance

American Elm, Ulmus americana, flowering

Many of these elms require considerably more pruning and training than other landscape trees, and the first 15 years often determine how they will perform for the remainder of their lives. In the case of elms, a small investment in maintenance during the "formative years" will have a huge payoff when they are approaching maturity.

Like most trees, these elms are best maintained with a strong central leader (where small limbs are pruned and the main trunk is allowed to grow tall). This ensures a straight stem and keeps the tree growing up rather than out. As the lower side branches grow and increase in diameter, they should be removed until the desired clearance for the site is reached.

Knowing when and how much to prune and maintain trees requires experience, so if you are not sure how to work on young elms, contact an experienced tree care professional to get you started right. The investment made now will pay off when your tree is growing beautifully, is structurally strong, and is providing shade on your property.

What Is Dutch Elm Disease?

Elm Bark Beetle

A fungus, Ophiostoma, that causes Dutch elm disease grow and reproduce only within elms. At times they are parasites, feeding on living tissue of the elm tree; at other times they are saprophytes, getting nourishment from dead elm tissue. The fungus is spread by the elm bark beetles.

Dutch elm disease is a vascular wilt disease. The earliest external symptoms of infection are often yellowing and wilting of leaves on individual branches. These leaves often turn brown and curl up as the branches die, and eventually the leaves may drop off. Although initially only a part of the tree crown may be affected, symptoms may progress rapidly throughout the crown.

Highly susceptible trees often die in a single year, but others may linger for several years. Symptoms progress quickly and death may occur rapidly in trees infected in early spring, while trees infected later in the summer may survive longer.

If the bark of infected elm twigs or branches is peeled back, brown discoloration is seen in the outer layer of wood. This discoloration in the xylem actually occurs before the foliar symptoms described above are seen; foliar symptoms result when sap flow ceases in the infected wood. Xylem browning is often discontinuous. In cross section, it appears as a circle of brown dots or a ring.

Is There Treatment For Dutch Elm Disease?

Cultural Strategies

American Elm, Ulmus americana, tree trunk

Today, some communities maintain active programs to manage Dutch elm disease because they have found that it is cheaper to manage the disease than to remove the large dead trees that it leaves behind. Some communities focus on cultural practices for disease management, including the avoidance of monocultures of elm trees, the removal of all dying or recently dead branches, trees, and cut wood (sanitation), and the breakage of root grafts between adjacent elms

Chemical Strategies

Insecticides. In the past, insecticides were sprayed on elm trees in attempts to kill the Beetle vectors of Dutch elm disease. This management strategy was expensive, not very effective

Fungicides. More recently, fungicides have been injected into trees infected by or at risk of infection by the Dutch elm disease pathogens. These systemic chemicals are most effective if they are used to prevent new infections or to prevent the movement of the fungi into parts of a tree that are not yet colonized. Several different fungicides have been used, but all are relatively expensive, and none is completely effective.

Breeding For Resistance

The long-term solution to Dutch elm disease lies in the development of disease-resistant cultivars of elms. Several Asian elm species have moderate to high resistance, and breeding programs in both Europe and the U.S. have introduced resistance from these species into native elm species. Other programs have focused on identifying and cloning individual American elm specimens that have moderate resistance to Dutch elm disease.

Further Information:

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