Dutch Elm Disease
Dutch elm disease is caused by the fungus Ophiostoma novo-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. Most elm species are susceptible to this disease.
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.
For more information see this bulletin about Dutch Elm Disease.