ISA Michigan, the local chapter of the International Society of Arboriculture, and the Michigan Oak Wilt Coalition have designated May as Oak Wilt Awareness Month.

Oak wilt is a vascular tree disease that can infect both forest oaks and oak trees in our landscapes. Over the last few years new “pockets” of oak wilt have been identified on landscapes throughout southeast Michigan, such as, Holly, Clarkston and Highland. Oak wilt is probably present in many other areas of southeast Michigan, but isn’t being correctly identified.

Texas and Minnesota have been fighting oak wilt for decades. Their experience dictates that prevention is the key to keeping oak wilt out of an area and thereby saving oak trees. Once an oak in the red oak group becomes infected it will only be a matter of time before it succumbs to the disease. Any other nearby oak trees can then easily become infected through root grafts. Arborists, foresters, utility crews, landscapers and homeowners should all become familiar with oak wilt so we can minimize its spread in Michigan.

What is Oak Wilt?

Oak wilt is a fungal disease that attacks the water-conducting system of oak trees. To block the spread of the fungus, an infected oak tree produces gums and resins which plug the water-conducting vessels of the tree causing the tree to wilt and die. The overreaction of the disease defense system of the tree ends up causing the demise of the tree.

It’s important to note that oaks are divided into two groups: the white oak group and the red oak group. Species in the white oak group are more resistant to oak wilt and on occasion have been known to recover from an oak wilt infection. Trees in the red oak group, however, will eventually die once infected with oak wilt. The two most common landscape trees in Michigan in the red oak group are pin oaks and Northern red oaks.

How Does Oak Wilt Spread?

There are three ways oak wilt can be spread to a healthy oak:

1. The least common method is via infected pruning tools. If a pruning tool is used to trim or remove an infected oak and next used on a healthy oak it is possible to infect the healthy tree. Disinfecting pruning tools between oak trees is a standard practice of knowledgeable and reputable tree services.

2. Insects, specifically picnic beetles, can transmit oak wilt to a healthy oak, but only if there is a fresh pruning wound or broken branch on the tree.

3. The easiest way oak wilt moves from infected to healthy trees is through root grafts.

Preventing the Spread of Oak Wilt

Unless there is already oak wilt on your property or on an adjacent property it is fairly easy to prevent oak wilt from getting started – don’t prune when picnic beetles are active. This is primarily in the spring and summer, but could be anytime temperatures exceed 40-45 degrees. An easier way to think about this is: Don’t prune oaks during the growing season.

On occasion, however, it might be necessary to prune an oak during the growing season. This might be due to repairing summer storm damage, discovery of a hazardous limb, or pruning to accommodate construction activities. If this situation does arise then tree wound dressing or latex paint should be applied immediately after the pruning cut has been made. This will limit the oozing of sap that attracts the beetles that spread oak wilt.

Controlling the Spread of Oak Wilt in Infected Areas

Once a single oak in an area is infected with oak wilt the disease can quickly spread to nearby oaks via root grafts. Roots of the same oak species have been known to graft on trees up to 50 feet apart.

Containing the spread of oak wilt once an epicenter has started is when the costs can start to mount. Before anything else is done the root systems of infected trees, and potentially infected trees, must be severed from trees with a higher likelihood of not yet being infected. Cutting down a tree infected with oak wilt before root severing is done may speed up root graft infection of healthy neighboring trees.

In most cases, trenchers or vibratory plows are used to create two rings at least five feet deep where roots are severed. The first ring is between the infected tree(s) and potentially infected trees and a second ring between potentially infected trees and trees that are probably not infected.

In urban settings all manner of infrastructure obstacles must be taken into consideration before trenching can start. Driveways, sidewalks, patios, property lines, buried utilities, irrigation systems, etc., could all be in the path of the ideal trench line. The roots that must be severed to stop the spread of oak wilt may be under, over or wrapped around any of these items.

Summary

Learning about oak wilt and knowing the species of trees on your property is the first step in preventing an oak wilt epicenter from getting started. If you think you have an oak that has been infected with oak wilt contact an arborist or your county extension agent immediately. Delaying action will only give the disease more time to spread to neighboring oaks, thereby increasing tree loss and containment costs.

Learn more about oak wilt or schedule a tree inspection with one of our arborists by visiting: http://www.owentree.com/Oak_Wilt.php