Dutch Elm Disease is a fungus that affects American elm trees in the United States. It is considered the most devastating shade tree disease to affect our forests and urban canopies in our country's history. Dutch Elm first appeared in Ohio in the 1930's, originating from Asia via Europe. The fungus is spread by both the elm bark beetle or root grafts between elm trees. The beetle is the most effective way to spread the disease. When the beetle comes into contact (feeds on) an infected tree, the fungus spores go with the bug to the next tree. The beetle prefers to feed in branch unions and once a wound is created by the feeding beetle, the spores can infect the new tree.
Some of the symptoms that are common with DED is yellowing or browning leaves (flagging) that stay on the branch. Once flagging begins, it usually progresses down the branch towards the stem. This progression down the tree can happen very quickly, so detection, treatment and aggressive pruning are the only way to save the tree once infected. If infection happens through root graph, treatment will not be affective at stopping the fungus.
The USFS offers extensive information on defense and treatment agains Dutch Elm disease in How to Identify and Manage Dutch Elm Disease.
Since the introduction of Dutch Elm Disease, there has been a lot of focus on creating a resistant tree because elms are such great urban trees. Elms feature unmatched growth rates (really fast) and toughness, both of which are helpful for survival in our harsh urban environment.
If you have an elm, you know how magnificent they can be. Fortunately, there are many hybrid varieties resistant to Dutch Elm disease to choose from, though some have greater resistance to Dutch Elm disease than others. We have planted the Valley Forge with much success. When deciding which hybrid to choose from, make sure its an American elm variety as opposed to Asian varieties; the form and appearance of American elms tend to better mimic the native elm that we are all used to seeing. If you have an American elm variety in your landscape, always have it pruned in the winter (between Oct. 15 and April 1st) when the beetle is dormant to minimize the chance of infection.
Fungicides do work as a preventative measure against Dutch Elm Disease. If you have an American elm that plays an important role in your landscape, we recommend serious consideration for treatment.