A fungus called Ophiostoma ulmi that was introduced to the U.S. in the early 1930’s causes Dutch Elm Disease (DED). The American elm (Ulmus americana) is highly vulnerable and the disease has killed hundreds of thousands of elms across North America. All native elms are susceptible, as are European elms. However, the Asiatic elms, (U. parvifoli) and Siberian elm (U. pumila) are highly resistant to the disease.
The DED fungus is spread by two insect vectors: the native elm bark beetle (Hylurgopinus rufipes) and the European elm bark beetle (Scolytus multistriatus). The fungus is transported on the beetles from infected trees to healthy trees as they feed on twigs and upper branches. The beetles lay their eggs in the bark and wood of stressed trees along with elm firewood with the bark left on. Developing larvae form channels just under the bark and the fungus grows through the galleries until it reaches the tree’s water conducting cells, or xylem. Chemicals manufactured by the tree during its effort to fight the disease plug up the xylem, causing the tree to wilt. In the Midwest, beetles typically have two generations per year.
DED is also transmitted through root grafts. A root graft happens when the roots of two trees intermingle and touch. Root grafts between trees are especially widespread in cramped urban street trees. Driveways and sidewalks are usually not effective in blocking root grafts, however, the disease usually does not spread in this manner beneath roads because road foundations are much deeper.
During the early summer is when effected trees are the easiest to identify. Leaves on the upper branches will curl and turn a gray-green or yellow and finally, crunchy brown. This symptom is called “flagging”, although a flag alone is not complete assurance that the tree has DED. Another symptom is brown streaks in the sapwood beneath the bark of affected branches, which is the blocked xylem. However, only laboratory isolation and identification can positively confirm that the tree has DED. Check with your local extension or State University, usually they will perform this test for a nominal fee. Most arborists find these two symptoms are enough evidence to treat or remove an elm.
There are two other diseases that may look like DED, Elm Yellows and Bacterial Leaf Scorch. Below is a symptom checker:
|Dutch Elm Disease||Bacterial Leaf Scorch||Elm Yellows|
|Caused by fungus||Caused by bacteria||Caused by phytoplasma|
|Affects individual branches first. Affects lower crown nearest root graft.||Damage initially observed on single branches, and spreads to entire crown; oldest leaves affected first.||Affects the entire crown at the same time.|
|Leaves wilt and turn yellow, then brown.||Leaves brown along margin, with a yellow halo.||Leaves turn yellow and may drop prematurely.|
|Symptoms often observed in early summer, however, could be anytime during the season.||Symptoms appear in summer and early fall.||Symptoms visible from July to September.|
|Brown streaking in sapwood.||No discoloration in sapwood.||No discoloration in sapwood.|
|No discoloration in inner bark.||No discoloration of inner bark.||Tan discoloration of inner bark.|
|No wintergreen odor.||No wintergreen odor.||Wintergreen odor in inner bark.|
- Branches infected with DED should be removed the same year the infection started. All infected branches should be pruned at least 5 feet, preferably 10 feet, below the last sign of streaking in the sapwood. Dip pruners often (best after each cut) in a solution of 10% bleach to prevent spreading the disease. Be sure to remove infected branches before the disease has moved into the main stem of the tree.
- Trees with many branches infected with DED should be removed. There is no cure. The best thing to do to stop the spread of the disease is to promptly remove the tree.
- Wood from DED infected elm trees need to be buried, burned, debarked, or chipped. When chipping and composting, temperatures must attain at least 120F. Cut logs from diseased trees should not be stored for firewood unless it has been debarked and there is no evidence of beetles.
- Neighboring elm roots need to be severed with a vibratory plow or trencher before the infected tree is removed in order to prevent the movement through root graphs.
- Choose cultivars that are resistant to Dutch elm disease. ‘Frontier’, ‘Homestead’, and ‘Valley Forge’ are a few that are offered in my area, but there are many more.
- Healthy elms can be treated with a preventative fungicide injection to protect trees from infection by beetle feeding. Although, fungicide injections are not effective in averting infection through root grafts. Injections can only be done by a trained arborist and depending on the chosen fungicide, must be repeated on a 1-3 year cycle.
- The consensus on treating the beetles with insecticide is not to. Contact insecticides require repeated applications during the growing season that may kill beneficial or harmless insects. Sanitation is by far the best way to control beetle populations.
Sadly, this is a prime example of what happens when we plant a monoculture of trees. Diversity is where it’s at!!
© The Naturarian