Oak wilt: what to know about the fungal disease

Oak wilt is a fungal disease that spreads during the summer. Prevention is better than treatment. Avoid wounding oaks from mid-April through mid-July.

Oak wilt is an exotic pathogen that has been around the Midwest for decades.  In many areas of Michigan and Wisconsin, the slow but certain spread has reached epidemic proportions. 

Similar to Dutch elm disease, oak wilt is fatal. There are very few survivors among the red oak group. 

The disease first enters an oak stand from small insects that carry the disease spores from a nearby infected stand. Once within a stand, the fungus spreads through the root systems. Species in the red oak group tend to graft roots between trees, an avenue exploited by this disease. Trees in the white oak group don’t typically display root grafting and their wood structure slows the internal spread of the fungus. 

At some point during the growing season, an infected tree will lose all its leaves within a few weeks. Any residual green branches or green sprouts mean something other than oak wilt has attacked the tree. Oak wilt fungus requires live tissue to survive. So, during the following spring and summer, after the disease kills a tree, then the fungus will produce a fruiting body. These gray lumps tend to push up the bark forming cracks. With a hatchet, the fruiting body can be exposed. It’s these fruiting bodies that attract sap-feeding insects that can spread the spores to healthy trees. They can also be transported with firewood. 

If oak wilt is suspected, it should be confirmed before taking any action. There are other health issues that can resemble oak wilt. 

Once the disease has infected an oak stand, there are two choices: the stand can be treated, which is expensive and sometimes difficult, or the disease can be left to run its course and serve to infect other stands.

Treatment involves isolating the infected root system, separating it from the rest of the stand to a depth of five feet, and then removing all the oaks within that perimeter. Determining the proper distance between dying trees and healthy trees is a function of tree diameters. The perimeter of the affected area is determined by these distances. Large oaks result in larger areas to be treated, as they have larger root systems than smaller oaks. Once the perimeters have been established, a vibratory plow can be scheduled during the fall. The plow severs the root systems, isolating the disease pocket.

So, how successful has this effort been? 

For the most part, the treatment appears successful in most places, so far.  Each treatment line needs to be monitored for at least three years after plowing. 

However, there have been locations where the disease has remained despite treatment. In some places, inoculum from nearby untreated stands can infect healthy stands next to treated areas. Freshly infected trees can be overlooked because they don’t show symptoms until the following year. Sometimes, large underground boulders, high bedrock or other obstructions, can prevent complete trenching, despite the diligent and conscientious efforts by contractors. Other times, there may have been an error in setting the distances between infected and healthy trees. And then, the distance charts are designed to be 95 percent certain, not 100 percent. 

While treatment cannot guarantee containment of the disease, lack of treatment will guarantee the death of all oak within a stand. Doing nothing will have an impact. So, the decision of whether or not to treat oak wilt depends on the value a forestowner places on the oak resource, and how well a forestowner might want to get along with neighbors. Partnering agencies remove most of the financial obstacles, when funding is available. However, the final decision belongs to the landowner.  

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