Many elm samples have been submitted to the Plant Disease Clinic in the last few weeks that have tested positive for Dutch elm disease.
Dutch elm disease affects all elm species. Dutch elm disease is caused by the fungus Ophiostoma ulmi, which invades the water-conducting vessels of elms. The leaves of trees wilt, turn yellow or brown, and then fall. Another diagnostic feature is the formation of brown or green streaks in the infected sapwood. This discoloration is visible when the bark is peeled back on symptomatic branches.
The fungus is spread by elm bark beetles which feed on and breed in elm trees. The fungus also can spread to adjacent healthy elms through grafted root systems.
Sanitation, the removal and proper disposal of all dead or dying elms, is the key to successfully managing DED. This involves early identification of the disease and immediate removal of infected elms. Prompt debarking, chipping, burning, or burying elm wood makes the wood unsuitable for beetles. Management may also include root graft disruption, the breaking of root connections between diseased and healthy trees to prevent the fungus from spreading from tree to tree through root grafts.
Therapeutic pruning is the removal of infected portions of an elm and is effective only if the elm has been inoculated by beetles. If the fungus is in the main stem or has come into the tree through grafted roots, pruning will not be successful. Systemic fungicides, when properly injected into elms inoculated by bark beetles, may save trees in the early stages of Dutch elm disease when less than 10 percent of the crown has wilted. These chemicals are injected into the root flares and are translocated throughout the tree. These fungicides can be injected into healthy elms to protect them from infection. The label states trees will be protected for approximately 3 years. Unfortunately, injection is expensive.
Dutch elm disease can be confirmed by submitting samples to the Plant Disease Clinic. Collect 3-4 branch segments from areas that have recently developed wilt symptoms. Branch samples should be approximately 1/2 inch in diameter and 6-12 inches in length. Bring samples to your county Extension office or mail them directly to the Clinic (323 Bessey Hall, Ames, IA 50011). There is a $5.00 charge for this test.
This article originally appeared in the July 19, 1996 issue, p. 124.