Dutch Elm Disease
Janna Beckerman, Assistant Professor, Department of Botany & Plant Pathology, Purdue University
The fact that emerald ash borer is a problem today is due to the fact that Dutch elm disease forever changed the landscapes of the Midwest and Northeast United States. Dying elm trees were often replaced by green ash, setting the stage for the current epidemic of emerald ash borer (EAB). Why not replace some of your dying ash with Dutch elm disease resistant elms?
Dutch elm disease (DED), caused by the fungus, Ophiostoma ulmi, causes wilting symptoms that ultimately results in the dieback and death of the infected tree (Figure 1). DED is most severe on American elm, and infects other native elms, like slippery bark elm (also known as the red elm), winged elm, cedar elm, September elm and rock elm. However, there are DED-resistant elms! Asian elms, like the Japanese elm and Chinese elm, and the Japanese Zelkova (Zelkova serrata), are considered resistant. ‘Princeton elm,’ an American elm, has been widely used throughout North America. ‘American Liberty’ is actually a collection of six, DED resistant elms marketed as a single variety, to increase landscape diversity. All of these trees are excellent choices for the urban landscape. There are several hybrid elms, which have outstanding disease resistance, but lack the vase-like form of the American elm. These include ‘Homestead’, ‘Pioneer,’ and ‘Sapporo’s gold’, to name but a few. ‘Cathedral’ is a hybrid elm with a lovely vase-like form.
Unfortunately, resistant is not the same as immune, which means these trees can and will get the disease, especially coming off a severe drought like last year. And that is probably what is going on with this Japanese Zelkova (Figure 2), and roadside elms across the state. Dutch elm disease, when caught early in the urban landscape, can be pruned out, and trees can be successfully treated with fungicide injections that will protect them for several years. However, a long-term successful program requires the use of community wide sanitation programs and good cultural practices, including keeping trees well watered during drying spells. Using these DED-resistant trees breaks up the monotony of Bradford pear, and creepy ubiquity of ‘Autumn Blaze’ and ‘Red Sunset’ maple. Using DED-resistant elms increases landscape diversity and limits any future ‘Nightmare on Elm Street’ to fiction rather than fungi—or beetles!
For more information on Dutch elm disease resistant elms, see Dutch Elm Disease and Disease Resistant Elms (pdf file)
For some wonderful photos of American elm, see The Princeton American Elm Blog
Click image to enlarge
Figure 1. Dieback and death of elm caused by DED
Figure 2. Roadside Japanese Zelkova weakened by drought is more susceptible to DED
Figure 3. Ophiostoma ulmi fungus in culture
Photo courtesy of Tom Creswell.