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1. How can I tell if my tree has OW or DED?

An oak tree with OW shows signs in June-September during leaf-on conditions. The first symptom is yellow or brown curling leaves moving from the branch tips down toward the main stem, also called “flagging.” The leaves may also start curling with a dark water-stained olive-green color. Next, the leaves drop, like fall leaf drop but occurring during the summer. The disease can kill a red oak rapidly, sometimes within two weeks. The process is usually slower in white oak species. 

Symptoms of DED in elm trees often first appear in late spring and early summer but can occur any time during the growing season. Leaves on one or more branches in the outer crown of the tree will turn yellow before curling and turning brown. Look for fallen leaves strewn on the ground during spring or summer.

If you would like more information on these diseases, visit the University of Minnesota webpage on oak wilt or Dutch elm disease. If you see any of these symptoms on your oak or elm trees and would like an inspection, contact Forestry staff by email at LAShadeTreePest@lakevillemn.gov.

2. What can I do to protect my oaks from oak wilt?

The best way to protect oak trees is to avoid pruning or wounding oak trees/roots between April and June, the highest risk time for new infections. Best practice is to prune oaks between November and early March, during the time that carries “no” risk for oak wilt transmission. To see if it is safe to prune oak trees, visit the University of Minnesota oak wilt webpage.

If there is an oak wilt pocket near your property, control measures can be taken to help limit the spread to your trees. Common control measures include the separation of root grafts between like-species oaks, chemical injections, and good sanitation practices. Root graft disruptions and chemical injections of healthy trees should always be done before the infected tree is removed. Keep in mind that OW does not discern property lines and often the best solution is to work with neighbors to stop the spread of the disease.

If you would like more information on OW, visit our Oak Tree Diseases webpage.

3. What can I do to protect my elms from DED?

The best way to control DED if it is already confirmed in your tree is through good sanitation. Remove diseased and dying elms and elm wood piles by chipping the wood to disrupt the elm bark beetles’ lifecycle. Unfortunately, good sanitation occurs after the trees are already dead or dying.

If you have a high-quality landscape healthy elm and would like to preserve it, there is a preventative chemical injection that helps keep the disease from taking hold in the tree. Many tree care contractors offer preventative DED injections, which are performed in the summer and must be repeated every other to every third year.

Things to consider when deciding to inject your elm:

• Is the tree structurally sound and in good health?
• Does the tree shade a large portion of your yard and/or the east or west side of your home, offering potential energy savings in the summer?
• Does the tree add significant aesthetic and real estate value to your property?

If you would like more information on how to diagnose and manage DED, visit the University of Minnesota Dutch elm disease webpage.