Dutch elm disease is a fatal vascular disease that attacks American elms. Beautiful old American elm shade trees have been killed in entire states in the eastern part of North America over the last several decades. In Montana, many populations of this popular shade tree have been killed in some of our larger urban areas, and the disease is moving into smaller communities and rural areas.
To protect the American elms in your community, be alert for the first symptoms of the disease. The earliest symptoms are usually the yellowing and/or wilting of the leaves on a single branch, usually a branch in the upper crown. The symptoms spread rapidly, to progressively larger branches and eventually to the entire tree.
When a diseased tree is found, cut and burn the tree as soon as possible to prevent spread to other trees in the area. There is no cure, and pruning is not effective. The disease is caused by a fungus that survives the winter in infected and recently killed trees, stumps, and in recently cut brush and logs. It is carried to healthy trees by elm bark beetles (not the same as elm leaf beetles), which feed in branch crotches, transmitting the fungus. Spring applications of the insecticide methoxychlor have been used to help prevent infestations of elm bark beetles in areas where Dutch elm disease has been found. However, complete coverage of large trees, especially crotches in branches, is difficult to achieve, even for tree professionals, and protection may not last the entire season. Application of insecticides to tree trunks to kill pupating adults is ineffective because the vector of the disease in Montana, the European elm bark beetle, pupates higher in the tree, under the bark, and insecticides to not penetrate the bark. The native elm bark beetle that transmits the disease in some parts of North America, and which pupates in the lower trunk area, has never been found in Montana.
The disease can also be transmitted to adjacent trees through root systems. If diseased trees are found, trenches at least two feet deep should be dug between adjacent trees in an attempt to prevent spread through roots.
Preventative implants of fungicides can be effective for trees that are especially valued for aesthetic or historical reasons. The procedure is relatively expensive and should be done only by professionals who have received sufficient training in the technique. Small holes must be drilled in the tree for insertion of the implants. When done incorrectly, the tree can be damaged. When done correctly, implants have been shown to be safe and fairly effective in preventing Dutch elm disease. However, pockets of dead wood result each time a tree is injected or implanted, and over time the pockets coalesce, leading to dieback of portions of the tree. Injections are done by some arborists that entail pounding pipes into trees and pouring materials into the pipes. Avoid these companies. The procedure is totally ineffective and causes irreparable damage. It is also costly, and so is the ineffective annual follow-up "servicing". For more information on the use of the method, see the fact sheet in this series, "Tree Injections and Implants".
Pruning of healthy elms should be done in late fall or early winter to prevent attraction of elm bark beetles to the pruning wounds. Limbs which have been pruned from the tree should be burned or buried to eliminate breeding sites, or covered with tightly sealed clear plastic to keep beetles out.
Wood from diseased trees which have been cut down should not be saved for firewood because the larvae (the immature stage of the beetle) will continue to mature and emerge from the cut wood. The only effective method for preventing beetle emergence from cut wood is by bark removal, in which case the wood can be used as firewood. Applications of insecticides to the infested cut wood and covering with either clear or dark plastic to help fumigate the wood or cause heat to accumulate and kill larvae in the wood are not effective. Nor is it considered safe to burn wood treated with insecticides indoors.
When planting elms, select resistant elm hybrid species, or the resistant American Liberty elm. Species such as hackberry (Celtis spp.) look similar to elms but are not attacked by Dutch elm disease or elm leaf beetles. It may be wise to begin planting a variety of replacement trees in your community, in the event the disease continues to spread as it has done historically in the east. Being alert to the early symptoms of the disease can help slow progress of the disease among America elm trees in Montana.
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Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997