The Cause of Dutch Elm Disease
Dutch elm disease is caused by an aggressive fungus (Ophiostoma-ulmi) that kills elms regardless of their health. It is a tree disease and remains active as long as there are susceptible trees. The fungus invades the water transporting vessels and produces toxins which cause the tree to react defensively to block the advance of the fungus. The combination of the toxins and the defense mechanisms of the tree prevent the flow of water, which causes wilting and tree death.
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How Dutch Elm Disease spreads
There are two ways this fungus is spread.
1 ) Female elm bark beetles lay their eggs beneath the bark of dead or dying elm trees. If the elm is infected with Dutch elm disease the newly hatched beetles will emerge from the tree carrying the deadly fungus on their bodies. As the beetles fly to healthy trees to feed, they spread the disease.
- 2-3 generations of elm bark beetles hatch each year
- Thousands of beetles may hatch from a single tree
2 ) Dutch elm disease may also spread when roots from two or more trees grow together. When elms grow in close proximity to each other, their roots can come into contact and graft together. This common root system provides the fungus with a pathway to spread through an entire group of healthy elms very quickly.
- To protect trees, Rainbow Treecare uses a machine to cut a trench in the ground between elm trees.
- This prevents their roots from growing together, eliminating the possibility of transmitting disease via the roots.
Due to proactive measures such as sanitation and prevention treatments, cities such as Minneapolis, Edina, Richfield, Minnetonka, St. Louis Park and Bloomington are still home to large inventories of American elm trees. In fact, the Twin Cities boasts one of the largest American elm populations in the country.
To properly manage Dutch elm disease, four steps are generally required.
- Identify the disease by the “flagging”, or wilting of leaves typically located at the ends of isolated branches in the canopy. The disease moves down the limb and then into the mainstem.
- Remove infected branches from trees (if identified in time) or remove entire trees immediately, in order to halt further spread of the disease throughout the neighborhood.
- Isolate infected trees from nearby healthy trees by soil trenching, which prevents spread of the disease via root grafting.
- Protect valuable remaining elms with the use of approved Dutch elm disease fungicide, Arbotect 20-S.