What is Oak Wilt?

Although Oak Wilt has been around since the 1970s here in Grand Rapids, MI, not many people know what it is. So, what is Oak Wilt? Well, Oak Wilt is a disease of oak trees, mainly Red Oaks, caused by the fungus Bretziella fagacearum. This fungus moves through the xylem (the tree’s water transport system) and blocks it. Because of this blockage, the tree does not get the proper amount of water, which leads to wilting and tree death.  

There are positive cases of Oak Wilt in 24 out of the 50 states and Michigan is one of them. Oak Wilt positive cases are currently in 61 counties here in Michigan, Kent, Ottawa, and Muskegon included.  

Why is this important?  

Well, oak trees make up 10% of Michigan forests. We have 20 million acres of forest land in Michigan meaning that Oak Wilt can impact 149 million Red Oak trees across it. Although Oak Wilt kills Red Oak trees rapidly, it can also kill White Oaks, just more slowly.  

How Do I know If My Oak Has Oak Wilt?  

It is always best to have a tree expert, forester, or arborist stop out to look at your potentially problematic trees, but you can look for a few things yourself. Are the leaves wilting? This is a telltale sign but could sometimes be a result of other diseases and pests, so have it checked. Are the leaves a bronze or brown color during the summer? This should also be checked further. Are the leaves falling prematurely (earlier in the summer rather than the fall)? If so, have it checked further. 

 Are you noticing signs of pests or other environmental problems near the oak? This can make an oak tree more susceptible. The more stress the tree has, the more susceptible it is. You should look more specifically for Picnic Beetles, as they can carry the fungal spores from tree to tree. These beetles are present from March through November. Oak trees are most susceptible from March to September, with May to June being the peak time of infection. Another sign of infection is the Mycelial mat under the bark, on the sapwood. You should not remove bark from an oak tree during the susceptible season, unless necessary for diagnosis.  

How is it Spread?  

As previously mentioned, it is spread through Picnic Beetles that are present from March through November. These beetles move the fungal spores from tree to tree. The fungal spores come from the mycelial mat. These mats have a distinct odor that attracts Picnic Beetles. The mycelial mat grows and eventually matures, creating a pressure pad, that will break open the bark. Bark that is split, broken, or not present makes for easy access to the mycelial mat for the beetles. They collect the spores and transport them to other trees to start the process all over again.   

A Red Oak will typically die within 3 to 4 weeks of the first symptoms. The mycelial mat grows between six to 12 months after death. So, your dead firewood could be carrying a mycelia mat and spreading Oak Wilt. Therefore, there are many restrictions on why you can not transport infected firewood into campgrounds or across state lines. You are directly transporting the pathogen!!!  

Oak Wilt is also spread through the roots. Root transmission accounts for almost 90% of newly infected oaks every year. Root transmission can travel up to 39 feet from an infected tree.  

How Do You Know for Sure?  

The only true way of determining Oak Wilt is sending samples into a diagnostic lab. In Grand Rapids, we can send our samples to the MSU Diagnostic Lab. Their website walks you through the sampling instructions. Make sure to follow them directly!  

What Can I Do?  

To help prevent the spread of Oak Wilt, you can do many things.  

1st: Do Not Injure Oak trees during the susceptible season.  

2nd: Do not expose the sapwood during the susceptible season. This means no pruning or trimming!  

3rd: If you see a new wound on an oak tree, apply paint or latex paint of the wound to prevent fungal spores from entering or exiting through the wound.

4th: Since Oak Wilt spreads through the roots, you should disrupt the shared root areas. This can be done using a stump grinder. You should always create this buffer between unhealthy and healthy oak trees. It is a great prevention tool!  

5th: Remove all diseased Oak trees and some of the shared root systems to create better buffer areas.  

6th: Remove all diseased trees, of any species, to reduce the number of fungal spores in an area. Reducing this number will help protect all your trees from potential diseases and limits the amount of stress put on them.  

7th: Destroy all branches, trunks, and stumps of all infected trees. This means chipping the branches you can, stump grinding the stump, exposed roots, and shared roots, and having a tree service properly remove and dispose of the trunk.  

8th: Remove all oak trees that have had their sapwood exposed due to injury or storm damage as soon as possible to prevent the spread.  

For more information on Oak Wilt, please contact us directly or you can find more information at:  

Contact Us – Great Lakes Tree Service

Plant & Pest Diagnostics (msu.edu) 

Oak Wilt – US Forest Service Research & Development (fs.fed.us) 

DNR – Oak Wilt (michigan.gov) 

Oak Wilt Qualified Specialist Directory — MichiganOakWilt.org  

Look for Oak Wilt (arcgis.com) Interactive Map