Dutch Elm Disease

Evidence of Dutch elm disease (Ophiostoma ulmi) on an American elm (Ulmus americana).
Photographer: Joseph O’ Brien, USDA Forest Service, Bugwood.org
Dutch elm disease (Ophistoma ulmi) is a fungal disease from Asia, not Holland. Its common name comes from the fact that it was first described by Dutch scientists, not many years before it arrived in North America in about 1930.

It can affect all of our native elm species, and has swept through much of the United States, killing a majority of the elms. The fungus is moved from infected elm trees to healthy ones by spores carried on the elm bark beetle. The disease can also be passed from tree to tree by their interlocking roots.1

When the elm bark beetle comes to feed and reproduce in the trunk of a tree, it drops fungal spores, which the tree attempts to quarantine by closing off newly-infected parts of its circulatory system—effectively shutting off the flow of water and nutrients to any limb that depends on that part of the tree’s vascular system.

The visual result is a tree with individual dead branches with brown, wilted leaves. As the fungus spreads, this symptom eventually progresses throughout the tree and kills it.

One option in the battle against Dutch elm disease is to inoculate individual elm trees against the fungus—an expensive option at $100 per tree.

To decrease the risk of attracting elm bark beetles (which move the fungal spores from tree to tree), elm trees can also be pruned of their dead branches—but it should be done in winter when there is no smell of fresh sap to attract more elm bark beetles. Elm bark beetles are more attracted to weak and stressed elm trees, so watering elm trees during a dry summer helps deter the beetles.

Elms growing within 50 feet of one another risk infecting each other through their roots. Trenching around a tree exhibiting early symptoms will help to protect nearby trees. It is critical to the survival of other elms that trees infected with Dutch elm disease be immediately destroyed, so that the elm bark beetles cannot continue to reproduce in its trunk; some estimates suggest that, left in place, a diseased tree can release upwards of 400,000 beetles within two years!2