Mission is a community where neighborhoods enjoy the beauty and benefits of large, mature trees. Maintaining and preserving the urban tree canopy is an important priority for our Governing Body. Many of the trees we enjoy are mature oak trees which were popular for landscaping when many of our neighborhoods were first established. Unfortunately, mature oak trees can be susceptible to various diseases, and recently a handful of trees in the Milhaven subdivision have been diagnosed with oak wilt. Because this disease can spread, we wanted to share information about how best to identify, treat and prevent it here in Mission.    

What Is Oak Wilt? 

As the name implies, this disease is particular to oak trees. The disease is caused by a fungus (Ceratocystis fagacearum) that invades the water conducting tissue of an oak tree causing the tree’s vessels to become blocked and unable to allow the upward flow of water through the tree to the branches and leaves. This results in the leaves of the tree becoming wilted and dropping. Ultimately, the tree begins to die back.   

Trees in the red oak group (red, black, scarlet, shingle, and pin oaks) are more susceptible to this fungus than trees in the white oak group (white, bur, chinkapin, and swamp oaks). Trees in the red oak group often die within a few months after infection, while trees in the white oak group may not show signs of the disease for a few years, and once evident, may live for many years, slowly dying back. 

How is Oak Wilt Transmitted? 

Oak wilt is transmitted in one of two ways. The first is by “grafting” of the tree’s roots. Trees of the same species that are planted close together will grow their roots in such a way that they become connected, or grafted, with each other. In this way, a tree that has been exposed to the fungus will transmit it to an adjacent tree of the same species.   

The second way is through the transfer of the fungus spores from an infected tree to a healthy tree via insects, primarily the picnic beetle. Once a tree is infected, the fungus will begin to grow abundantly between the bark and the sap wood of the trunk or a branch. This growth, called mycelial matts, will eventually break open the bark releasing a fruity odor that attracts the beetles.  The beetle will pick up the fungal spores and then transport those spores to another tree that has exposed sap wood from an open wound.  In this way, the disease can be spread over distances of several hundred feet.  Oak trees are particularly susceptible to wound infections during the spring and summer months when the weather is warmer and sap flow from open wounds is greater. 

How Do I Know If My Tree Has Oak Wilt?   

For those trees that are in the red oak group, symptoms will first appear in late spring or early summer. Leaves at the top of the tree will begin to discolor, wilt, and drop. As the disease progresses, this discoloration and wilting of leaves will spread to the ends of the lateral branches.  Discoloration will consist of the leaves turning a dull, bronze color at the tips and along the outer edge. There may be a sharp line separating the discolored portion from the normal green portion of the leaf. Infected trees often wilt completely within several weeks of the symptoms first appearing.  

Trees that are in the white oak group will display similar symptoms but not until mid to late summer. Once symptoms do appear, the progress of the disease is much slower and can be over many months if not years.  

Trees may exhibit symptoms similar to that of oak wilt but not have oak wilt.  Anthracnose is a fungal disease that affects the leaves causing them to brown and drop but has little to no effect on the rest of the tree.  Drought conditions, waterlogged soil, nutritional imbalances, chemical injuries, lighting, and construction activity nearby are all things that can cause stress on a tree resulting in symptoms similar to oak wilt.  A trained arborist can best detect the differences between these issues and oak wilt.  You can also have your tree tested for oak wilt. 

How Can I Have My Tree Tested for Oak Wilt? 

The best way to do this is to provide samples from several different branches of your tree that are showing symptoms of the disease. The samples should be 6 to 10 inches long and ½ to 1 inch in diameter.  The branches should have freshly wilted leaves and must be living. It is also good to provide a few partially discolored leaves. Samples should be sealed in plastic bags and kept cool with ice packs during shipping. More information about testing and where to send the samples can be found on the Kansas State University’s Extension Program website.

What Can Be Done to Prevent Oak Wilt? 

Once a tree has been infected with oak wilt, there is little that can be done to ensure that it lives. The best course of action is to have the tree removed. This will require the services of a specialist that can remove the entire tree in a safe manner. Cleanliness of tools and work area is key to ensure that the disease does not spread to other trees during the removal process. Having the tree removed in the fall or early winter, when trees are going dormant, will also help to prevent the spread of oak wilt. 

Trenching around the drip line (the very edge of the lateral branches) of a healthy tree will cut any tree roots that may have grafted with a nearby, infected tree.  Again, this requires the expertise, tools, and equipment that only a tree service would have. 

Trees may also be treated with a fungicide that is injected into the base of the tree to prevent or suppress the oak wilt.  The best time to do this is between late spring and early fall. As with the other techniques, this will require the services of a tree service licensed and trained to handle the hazardous chemical.   

Preventing injury to a tree and proper pruning are the easiest and most effective ways to ensure that trees stay healthy.  Be careful when mowing and trimming the grass around the tree so as to prevent accidental cuts into the tree exposing the wood sap of the tree. If a branch is accidentally damaged during a storm event, cut it off at the point nearest the base and treat the cut with a wound treatment compound or paint. This will deter the picnic beetle from being attracted to the fresh wound and spreading the disease. It is also best to prune your tree in the late fall or early winter after it has gone dormant. This will also reduce the chance that the sap from the tree will attract insects.  

Tree Removal

It is best to contact an arborist that is able to diagnosis your tree and develop the best plan of action for taking care of your tree. Admittedly, the removal of a tree can be expensive.  If you feel that you cannot afford to do this, the City has grant programming to assist with the cost based on income eligibility. 

Additional Resources  

There is an excellent resource on oak wilt through the Iowa State University Extension and Outreach entitled Oak Wilt – Identification and Management. 

City staff can also provide additional information or assistance.  Please contact the Community Development Department at (913) 676-8350 or email Deputy City Administrator Brian Scott.