In the summer, Dutch elm disease (DED) can show up on susceptible American elm trees. At about the same time, the very common elm anthracnose fungal disease also appears. I occasionally receive inquiries from property owners who notice brown leaves on their stately elms at this time of the year and believe their trees have DED. Those trees may or may not be infected with DED. I will describe the differences in appearances of DED and elm anthracnose disease (Gnomonia ulmea) infected leaves. I have previously written about DED in American elms in this column. I will contrast the differences in the appearances of American elm leaves that have been infected with DED and those infected with the elm anthracnose fungus.
Elm leaves may discolour in August or earlier as a result of cooler nights, or through a common fungal leaf disease called elm leaf anthracnose. Many of our trees get infected with anthracnose, including ash, oak, maple and fruiting trees. Typically, this disease causes the leaf to develop yellow to brown spots or blotchiness against a green background. Sometimes, the leaf edges or margins can turn brown. There can be some curling of the sides of the leaf, but the leaf rarely droops and curls. The leaves tend to initially maintain their smooth, flat appearance, although as they age, they can become tattered and somewhat curled. The leaves will often develop a cluster of separated brown-grey spots. Anthracnose-infected leaves can stay on the twigs for a long time into the fall, even when dead. Leaves that have been infected with DED, however, droop and curl inwards. They will usually go through colour changes from yellow-green to yellow-brown and then to totally dark brown. Typically, these infected leaves will stay on the tree for a while until strong winds bring them down. At this time, there will be a noticeable bare patch, usually at the top of the tree, where all the leaves fall off.
There is no known cure for DED. Severe infections of the elm anthracnose disease can be treated with approved fungicides if required. In my experience though, fungicides are rarely necessary for anthracnose-infected leaves. Again, it should be stressed that this is not DED.
Repeated infections of anthracnose year after year can and do kill twigs and small branches in elm trees, as well as many other tree species, especially ash trees. Trees that are under environmental stress caused by such factors as severe soil infertility, disturbances to the soil surface area — especially back filling — and inappropriate treatment with chemical pesticides will likely die. Anthracnose can also be spread, especially along leaf margins, by the feeding of aphids and leaf mites.
The complexity of environmental stresses in all trees, not just elms, often requires the experience of seasoned arborists and tree experts who can readily sort out these complex issues by virtue of their knowledge and years of training.
American elms can be beautiful trees. The presence of brown leaves does not necessarily mean they have been infected with DED. In most instances, the causal agent is elm anthracnose.
For those who want to know more about anthracnose disease in other tree species, I have written extensively about them in my book, Dr. Tree’s Guide to the Common Diseases of Urban Prairie Trees, published by Xlibris, 2014.
Michael Allen M.Sc.F., RPF (ret’d) is a consulting urban forester, tree diagnostician and certified arborist. He owns Viburnum Tree Experts. He can be reached at 204-831-6503 or 204-223-7709. His website is treeexperts.mb.ca.