Dutch Elm Disease
Dutch elm disease (DED) kills our native American elm (Ulmus americana), red elm (Ulmus rubra), and rock elm (Ulmus thomasii). The non-native Siberian elm (Ulmus pumila) is more resistant to the disease, but can also succumb to it.
The City of Maplewood manages DED through inspection, sanitation and education. The city contracts with a state-certified tree inspector to identify and mark elms with DED. When an elm with DED is found, the property owner is sent a letter informing the owner that the tree is diseased and removal is recommended.
Causes, Signs & Spread
DED is caused by a fungus, Ophiostoma novo-ulmi. The fungus is transported to healthy elms by bark beetles. When the fungus invades an elm, it begins to grow in the water-conducting vessels (xylem). The tree responds by trying to wall-off the fungus. The obstructions and fungus block the effectiveness of the water movement in the tree. This blockage leads to the wilting and eventual death of the affected tree.
Because the fungus obstructs water movement to the leaves, the first sign of DED is the yellowing and wilting (flagging) of an isolated branch in the upper canopy. The leaves dry out quickly and turn a dark green or brown, and may fall or remain on the dying branch. The branch progressively dies back, and the entire tree can die within weeks.
The fungus that causes DED is spread by both the native elm bark beetle and the European elm bark beetle. An adult beetle can leave a DED-infected tree with fungal spores on its body. It flies to another elm, and begins to feed in the upper canopy. As it bores into the wood, it injects the tree with the fungus, which enters the tree's water vessels. Adult beetles will also lay eggs under bark of recently dead or dying elms. The eggs hatch, and the larva feed just below the bark, picking up fungal spores in infected trees.
In addition to being spread by beetles, DED can spread by the fungus traveling underground through root grafts. Transporting diseased firewood that harbors elm bark beetles may also spread the fungus.
DED is managed by quickly removing affected trees that harbor the fungus and the insects that transfer DED to other trees. Root graft disruption and chemical treatment are sometimes also used to fight the disease.
- Chemical Treatment: An injection of a systemic fungicide can help a healthy elm resist DED, and may sometimes help a newly infected elm survive. The fungicide is injected around the base of the tree, and should be done by a licensed contractor. The treatment must be repeated every three years.
- Diseased Tree Removal: Trees with DED must be removed. If the city marks a diseased elm on private property, the property owner will receive a letter from the City of Maplewood informing the owner that the tree needs to be removed, usually within 3 weeks. If the marked tree is on public property, such as a park or boulevard, the city (or other government agency that owns the property) is responsible for the tree removal. If a stump remains, it needs to be de-barked, so habitat for bark beetle does not remain.
- Root Graft Disruption: Elms growing within about 50 feet of each other can develop a common root system as their roots graft together. Because of the root grafting, DED fungus can travel through the roots into healthy elms. Root graft disruption, or the severing of root connections, is done using a vibratory plow with a 5-foot blade. This can stop the disease from spreading underground, but must be repeated every three years. The diseased tree must still be removed, and signs of disease spread need to be carefully monitored.
Please carefully weigh the pros and cons when considering injection. Injections are costly and must be repeated, they are not always effective, and they can have negative environmental impacts.
Storing & Burning Diseased Elm Firewood
Elm firewood can be stored only if the logs are de-barked. A fireplace log can harbor hundreds of elm bark beetles. De-barking the log removes the habitat and moisture level the bark beetle requires.
An injection of fungicide may help protect a healthy elm from DED. A tree service contractor can be hired to inject a chemical fungicide around the base of the tree. This procedure must be repeated every three years. The city tree inspector can help determine if the tree in question is a good candidate for preventive treatment.
Saving a Diseased Elm
If DED is noticed at the first sign of wilting, it may be possible to save the elm by removing the wilted branch. This is followed by a fungicide treatment done by a tree service contractor.