Forests
Prevention
Oak wilt can be very expensive to control, but prevention is effective and cheap. If an oak stand is in the high-risk zone (see map on our Oak Wilt main page), and if oaks are a desired future component, avoid harvesting, cutting firebreaks, road construction, and road maintenance in or next to the stand from April 1‒July 15.
If the spring is unusually warm, the risk of aboveground oak wilt infection can occur before April, but this is rare. If the daily high temperature is 60 degrees Fahrenheit or warmer for about one straight week, there may be a risk of oak wilt. Aboveground infection risk has started in March once in the last 15 years. That was in 2012, the warmest March in the state’s modern history.
For more details on prevention, download the Minnesota DNR Oak Wilt Guide
Abundant oak wilt in woods
In woods with lots of oak wilt, controlling it is often unrealistic. In most cases, we recommend focusing on creating a healthier, more resilient forest for the future. Frequently, that involves planting or encouraging an array of native trees growing in canopy gaps (e.g., in areas where oak wilt has killed oaks), protecting saplings from deer browse, and ensuring that competing shrubby vegetation (e.g., common buckthorn or hazel) is not shading saplings. Young oak seedlings and saplings that grow from acorns cannot get oak wilt infection through roots, and they are an important part of the ecosystem. We recommend encouraging or planting multiple oak species and other trees well-adapted for the site.
Controlling oak wilt
Two or three steps are needed to control oak wilt. The fewer and smaller the number of oak wilt pockets, the more likely control will be successful. Consider working with a forester to combine disease control with other stand improvement treatments. The following generalized control steps are important to do in the order listed. For details and options, read the Minnesota DNR Oak Wilt Guide.
- Stop the underground movement of the disease by severing or killing roots grafted together creating what is called a treatment boundary. For this step to be successful, several “healthy” oaks surrounding the pocket of wilting or dead oaks usually need to be sacrificed because they likely already have oak wilt in their root systems. The treatment boundary installation usually occurs from August through early winter, depending on the method used. Note that fungicide injections do not stop the underground movement of oak wilt.
- (Optional but recommended) Remove the healthy-looking oaks within the treatment boundary, since they will likely die from oak wilt in the next few years. This removal is called cutting to the line.
- In yard settings where homeowners are unwilling to preemptively remove healthy oaks within a treatment boundary, there are 3 options: (i) When and if the trees die, remove and destroy them before the following April. (ii) Keep them alive by injecting them with propiconazole every other year for several years. (iii) Protect them with a secondary barrier line. These secondary barrier lines often fail, so we do not recommend this option.
- Lastly, remove diseased oaks to stop the above-ground movement of the disease. It is crucial to remove diseased oaks only after making a treatment boundary. Properly handle infected wood before April. If you cannot make a barrier line, due to expense or lack of equipment, remove diseased red oaks after the ground is frozen, and properly handle the diseased wood before the next April.
Yard Trees
Prevention
Oak wilt can be very expensive to control, but prevention is effective. To prevent oak wilt, do not wound oaks from April through July, which is when oaks are most susceptible to infection. This is particularly important in the high-risk area (see map on our Oak Wilt main page). If hazardous branches or trees must be cut during this high-risk period, immediately apply a tree wound paint or shellac to the cuts (or the outer sapwood of a stump in the case of entire tree removal). Wounds may be susceptible to infection for up to five days.
During an unusually warm spring, the risk of oak wilt can occur before April, but that is rare. It has only happened once in the last 15 years. If the daily high temperature is roughly 60 degrees Fahrenheit or warmer for about one week, oak wilt infection risk can start.
An oak being injected with fungicide to prevent oak wilt. Photo by Joseph O'Brien, USDA Forest Service, Bugwood.orgIn yard settings where healthy oaks are extremely valuable to homeowners, and nearby oaks have oak wilt, homeowners may consider hiring an experienced, professional arborist to inject their healthy red or bur oaks with a fungicide containing propiconazole. Injections do not prevent infections through root grafts, but they will sustain an oak as long as injections continue every other year for the life of the tree. White oaks can be treated with propiconazole once they display early symptoms of oak wilt.
Control
In yard settings, hiring an experienced contractor to control oak wilt is highly recommended. Read generalized steps to control oak wilt above. For details and options, read the Minnesota DNR Oak Wilt Guide. Before investing in any control, we highly recommend getting a lab confirmation of oak wilt. This is especially true for dealing with bur or white oaks, since several other problems mimic oak wilt on those species.
How to properly handle infected wood
Tarp wood and completely bury the edges to exclude sap beetles.Logs and branches from infected red and bur oak may produce infectious spores in the summer, autumn, or spring following wilt. The spore production risk in bur oak is small and almost zero in white oak. Red oak branches under 6 inches in diameter also have a low risk for spore production. For red oak branches larger than 6 inches in diameter, the following options are recommended.
- Cut down infected oaks before April. Make sure you cut the stumps as close to the ground as possible. Burn, debark, or chip logs and larger branches before April. You may burn fresh oak wood in an outdoor boiler if you mix it with dry wood. Chips, bark, and twigs will not spread infection, so they can be left on site.
- Process diseased logs into lumber or kiln-dry before April.
- Tarp wood from April through July. Completely bury the edges of the tarp to prevent sap beetles from coming into contact with spores. The tarp should be thick enough to prevent punctures.
Diseased oak logs and branches will only produce spores once, so oak trees or logs seasoned over one year from tree death will not be infectious.
More details on handling infected wood can be found in the Minnesota DNR Oak Wilt Guide (PDF).