Dutch Elm Disease--an Epidemic Year

In years past it was said that, when driving through Iowa, you could see at least one grain elevator from almost every location in the state. In 1992, on the other hand, it's almost impossible to be out of view of a wilted or dying elm along the state's roadsides. Mortality of American elms and other native elms due to Dutch elm disease is the highest in many years. The disease is visible in fence rows, along rivers and streams, and in urban yards. Trees of all ages have been affected.

For latecomers to the Dutch elm disease (DED) story, let's review a few points. The disease is a vascular wilt, which means that it blocks the flow of water and minerals within the sapwood of the tree. The causal agent, a fungus, spreads in two distinct ways: from tree to tree by root grafts, and over longer distances by means of elm bark beetles. Because adjacent elms (within 50-60 ft of each other) often fuse their root systems together, the DED fungus passes directly from an infected tree to an adjacent healthy tree through their joined sapwood. Groups or rows of elms can wilt and die almost synchronously as a result of root-graft transmission. Two species of elm bark beetles are responsible for spreading the disease more widely. These tiny (less than 1/4" long) insects pick up sticky spores of the fungus in dying trees and carry them to healthy trees, where they enter through feeding wounds created by the beetles.

Management of DED is still expensive and arduous. The program MUST start with sanitation. This means that dead or dying trees need to be removed promptly in a town or neighborhood where they begin to appear. This practice greatly slows the progress of an epidemic by reducing the opportunity for the bark beetles to reproduce. By the same token, DED-killed trees should not be used for firewood unless the logs are debarked. Root grafts between adjacent trees can be broken by trenching or by fumigation. This is most effective if done promptly after the first flags are noticed in the crown of one of the trees. Injection of systemic fungicides can inhibit disease development for up to three years following the injection. Injections are usually done by a professional arborist, using compressed gas to force the material up into the tree. The most effective fungicide available, according to recent comparative trials, is Arbotect 20-S, manufactured by MSD Agvet Co.

Why are so many native elms dying this year? There is no certain answer. One plausible (but unverified) explanation is a cyclical peak in populations of the bark beetles.

The DED epidemic this year doesn't change the optimistic outlook for disease-resistant elms in Iowa (see HHPN article, June 10, 1992). With good genetic resistance and proper planting strategies, elms can be a part of our landscape in the 21st century.

This article originally appeared in the July 15, 1992 issue, pp. 1992 issue, pp. 123-124.


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