Symptoms of Dutch Elm Disease
The first symptom of Dutch elm disease is wilting or “flagging” in one or more of the branches, usually starting in the outer portion of the crown. Leaves on infected branches turn dull green to yellow and curl, finally becoming dry, brittle, and brown. The symptoms progress down the limb and eventually throughout the entire tree.
Trees infected through root grafts can die very rapidly, while trees infected via the feeding of bark beetles can take 1 – 3 years to die.
Another symptom of the disease is the discoloration of the water-conducting vessels (xylem) in the tree. Peeling the bark off infected wood easily reveals this staining.
Diagnosing Dutch Elm Disease
A few key distinctions will help in accurately making sure that you are in fact dealing with this dreaded disease.
- Leaves on infected trees are almost always curled or wilted looking. They often drop off.
- Symptoms progress as the fungus grows in the tree. Thus, the pattern of leaf death visible outside the tree reflects the fungus growth inside. Remember that the initial infection grows downward in a narrow band until it has reached the roots. It then spreads sideways and grows back up the rest of the tree. See the Technical Description of Dutch elm disease for more information.
- Check under the bark close to where there are external leaf symptoms. Dutch elm disease always causes the tree’s water-conducting vessels to turn a dark brown. Finding this discoloration along with wilting leaves is a very strong indicator that Dutch elm disease is present. Use a chisel and a hammer to open a hole in the bark to check for the discoloration.
- Trees in the spring can die rapidly – causing the appearance that the disease is moving very quickly. What is happening in reality is the fungus was in the tree from last summer, the tree grew new tissue over the top of the infected wood, and then the fungus colonized in the newly formed wood causing rapid dieback. Infections that happen in the current year can be seen reflected in leaf dieback as the fungus grows.