Dutch Elm Disease

Dutch elm disease is a serious fungal disease affecting elm trees. The disease was first introduced to North America in 1928 on imported timber. It is principally caused by two related fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi. Dutch elm disease typically spreads in one of two ways:

1. Elm bark beetles may carry the fungal spores between susceptible hosts at some distance.
2. Neighboring elm trees may form root grafts through which the fungus may spread.

The introduction of Dutch elm disease led to a massive epidemic in the elm trees of North America. The disease killed off a huge swath of American elm trees in the mid-twentieth century. Americans elms (Ulmus americana) were once a nearly ubiquitous species. Favored for their ability to form a neat, closed canopy, the American elm was a popular choice for planting along city streets. They are now a rarity in many parts of the country they once dominated.

Dutch Elm Disease in Texas

Dutch elm disease is an ongoing problem in the Northeastern and Midwestern United States. It is comparatively less common in Texas. Some speculate that Dutch elm disease may be less common here because the pathogen is sensitive to heat. This theory aligns with our experience—we see infections flare up most when temperatures are mild.

Dutch elm disease is also gaining a foothold in some pockets of north Texas. In particular, most infections are concentrated in the Mid-Cities region; there have been confirmed infections in Murphy, Sachse, Flower Mound, and Grapevine.

Symptoms of Dutch Elm Disease

Similar to oak wilt, the fungal spores spread throughout the vascular system of the tree. In an attempt to stop the spreading pathogen, the tree plugs the water-carrying vessels with balloon-like structures called tyloses. Unfortunately, this immune response does not occur quickly enough in susceptible species. The response is actually counter-productive. Rather than containing the fungi, the tree loses its ability to transport water up the trunk.

In Texas, the symptoms of Dutch elm disease are most visible when temperatures begin to rise—from mid-May to early June. The leaves on the outer crown wilt and turn yellow. Eventually they turn brown and may be shed from the tree. As the infection progresses, the yellowing and wilting advances along the infected branches from the outer crown down toward the trunk.

Another symptom of Dutch elm disease is brown streaks running lengthwise along the sapwood of wilted branches. This symptom is often missed because the bark must be removed in order to observe it.

Properly diagnosing Dutch elm disease can be tricky. In our climate, heat and water stress are common concerns. For this reason, Dutch elm disease may be mistaken for a lack of sufficient water. Positive diagnosis typically involves sending a branch sample to a plant diagnostic clinic.

Treating Dutch Elm Disease

Preventative treatments are the most effective way to lower the risk of Dutch elm disease. If you live in a pocket of infections, such as the Mid-Cities, it’s important to monitor your landscape and neighborhood for signs of Dutch elm disease. If the disease is a recurring problem in your area, consider preventative treatments every 1–3 years.

When branches start flagging due to wilt, we can begin “bark tracing” to remove infected tissue and, hopefully, contain the spread of the disease. Root grafts should also be broken by trenching in order to prevent the disease from spreading to neighboring elm trees. If symptoms have progressed and management is no longer possible, the infected tree should be removed and properly disposed of.

Additional Resources

The University of Minnesota Extension has a great general factsheet on Dutch elm disease. Minneapolis–Saint Paul still has a vibrant population of American elms in what would otherwise be a hotbed of infections. This is largely due to their excellent management strategies at the community level.

David N. Appel, Professor Emeritus in the Department of Plant Pathology & Microbiology at Texas A&M, wrote an excellent primer on Dutch elm disease in Texas some years ago. He discusses the nuances of the disease in Texas specifically.