History of Dutch Elm Disease in Minnesota

Our tendency has been to acquiesce to ease
and convenience, which means we’ve planted
easily grown, easily transplanted species

Minnesota has not lost all of its elms since Dutch elm disease was first found in the state more than 30 years ago. While it is likely that many elms in forested areas of the state will be lost, including those of any substantial size, elms are a tenacious species and will always be with us. We just won’t have as many of the large specimens. The cities, however, face a different scenario, and any community that has good judgement will be able to keep many of its elms.

A reasonable program of management can continue to maintain low losses and preserve a substantial portion of our shade tree elms. The management program is a wise investment because the absence of any effort to stop Dutch elm disease will result in dead trees which still must be removed.

An argument can be made that a community actually saves money by keeping the elms it has for as long as it is able. Cities should appreciate that it takes decades for newly planted replacement trees to reach the stature and beauty of our existing residual elms. And the new trees will have their own problems, including susceptibility to their own diseases. Finally, for the very reason that so many elms were planted in the first place, we have to acknowledge that few other tree species are as beautiful and acceptable in as many respects for a shade tree.

The choice of replacement trees is surprising in some respects. It’s understandable that we are planting more ash (26 percent) trees than any other single species because this tree is cheap to produce, easily transplanted, and, when young, reasonably attractive. But, older ash are not so attractive; they lose their leaves early in fall and are late producing leaves in spring. Some varieties of ash have had a high rate of mortality, and we now have two serious diseases of ash to contend with, ash yellows and Verticillium wilt.

Another 38 percent of our new replacement trees are exotics which are not entirely suited for this part of the country. They are often prone to disease and insect problems. Why plant untested exotics when there are dozens of native species? The answer in part is that Minnesotans are not growing all of their own trees. A high proportion are being shipped in from other parts of the country.

Even species native to Minnesota are not necessarily satisfactory if the seed from which they are grown comes from a distant tree. Red maples, for example, grow over much of the eastern United States, from Maine to Florida. But maples evolved and adapted to Florida are not likely to survive Minnesota’s climate. Other species that are being planted also have their own disease problems. Potentially, up to 20 percent of the honey locusts being planted can be infected by a lethal canker disease.

Why not resistant trees?

Nothing has been said about resistant varieties of elms. When Dutch elm disease was causing tremendous losses in Minnesota in the 1970s was not the time to develop resistant elms, a process which takes years. The Dutch have been at it for six decades with only minimal success. They are still searching for that elm they need.

Minnesota’s reforestry recommendations were based on planting trees which we know are resistant to Dutch elm disease. These include maples, hackberries, oaks, and birches to mention a few. There was no question about these trees being resistant to Dutch elm disease and they were suited to our climate.

Elm’s of any variety do little more than harbor the fungus and the beetles which spread it. The only appropriate use for so-called “resistant” varieties may be as firewood or mulch chips.

The resistant elms which were available in the early years of the Dutch elm disease epidemic, were exactly that, resistant but not immune. More important they were both not well suited to Minnesota and not comparable to the native elms in stature and beauty. Siberian and Chinese elms have resistance to Dutch elm disease fungus but are undesirable species because they are subject to winter injury. A not entirely facetious recommendation by this author has been that these trees be pruned at the ground line. It is not an uncommon scene in Minnesota to see rows and rows of dying Siberian elms which had been planted as windbreaks along roads and around farmers’ fields. Once established they can become a problem because of their heavy seeding habit.

Now is the time to choose carefully and wisely as we select the tree species we plant to renew our urban forests. Thus far this has not been done. Our tendency has been to acquiesce to ease and convenience, the translation of which means we’ve planted easily grown, easily transplanted species. Unfortunately this has led to planting too many ash trees, too many honey locusts, and too many little leaf lindens. There are other choices.

Is there a place for the elm in the future? The answer is a very definite yes. There are resistant elms more recently developed which have more desirable characteristics and these are now being evaluated in Minnesota. We should consider keeping the American elm in our landscape. As Dutch elm disease becomes less abundant, and possibly if the nonaggressive strain becomes more common (in 1977 only 8 percent of 1,124 isolates were nonaggressive), the American elm might be replanted in some locations. It should never be planted as abundantly as was done by our forefathers, but it should be a part of our landscape plans.

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