OAK WILT MANAGEMENT IN MICHIGAN
Oak Wilt is a devastating disease of oak trees. Many counties in Michigan have oak wilt reported and some serious outbreaks have occurred. As of this writing, we are unaware of any reports of Oak Wilt in Midland, Bay or Saginaw counties.
Some counties with significant outbreaks include Livingston and Oakland counties downstate and Ogemaw, Roscommon, Gladwin, Harrison, Clare counties mid-state and in the Traverse City area upstate.
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This disease is a vascular wilt similar to Dutch Elm Disease. It is caused by a fungus that plugs the vascular channels in the tree. The fungus is carried by a beetle that feeds on infected trees and then later on healthy trees, thus transferring the fungus.
When the infection occurs on a Red Oak, the tree usually dies within a month or so. In White Oaks, the disease is more indolent and may take several years to kill the tree.
PREVENTION OF OVER LAND TRANSMISSION OF THE DISEASE
The most important way to prevent oak wilt is to avoid injury to the tree during the warm months, especially April through August. So no pruning should be done during those months unless necessitated by storm damage. If pruning does take place, the trees should NEVER be climbed using climbing spurs and any wounds should be sealed immediately with pruning paint. These precautions are to avoid attracting the beetle that carries the disease.
Another method of disease transmission could be from contaminated cutting tools, so before trimming an oak, the saws, pruners, etc., should be disinfected with bleach water or alcohol.
PREVENTION OF SPREAD BY ROOT GRAFTS FROM INFECTED TREES TO HEALTHY TREES
If a tree is infected and other oaks are nearby, the following measures have helped to prevent spread of the disease through root grafts:
1) Infected tree should be removed as soon as possible, removing (not grinding) the entire stump and root plate with a backhoe if possible.
2) If stump and root plate are removed with a backhoe, one method of disposal would be to invert and bury in place. Otherwise the stump and root plate and all wood from the tree should be removed from the site or burned promptly. If the wood is not burned and is stored near oak trees, it should be covered with plastic tarps until burned.
3) If removing the stump and root plate is not practical or possible, then trenching or vibratory plowing between infected tree(s) and healthy trees should take place before cutting down the infected tree(s). The recommended depth of trenching or vibratory plow disruption of roots is 5 feet. However, 80-90% of roots are within the top two feet of soil, so a large portion of potential grafts can be removed with lesser depths of trenching.
FUNGICIDE INJECTIONS FOR HIGH VALUE TREES
Healthy trees can be injected with propiconazole as a preventive measure and has been shown effective in treating the problem in white oaks, slowing its progression in red oaks and preventing the disease in many healthy red oaks.
In an area with a lot of oak wilt, injections of high value trees can be repeated every two years. This is advisable if there are infected oaks in the area that beetles will be visiting and becoming contaminated and then possibly visiting healthy trees, even trees that are not wounded by pruning or storm damage.
In addition, theoretically, some oaks may have been inoculated with a small amount of fungal elements that may multiply in the future, so applying fungicide into apparently healthy oaks may stop these infections.
Propiconazole injection can be done by injection or infusion. The cost varies from 8 to 10 dollars per diameter inch for injection, depending on dose and method used. The macro-infusion method probably gives more thorough distribution through the tree, but is more time-consuming and thus more expensive (14 to 20 dollars per diameter inch.)
For high value trees in an area with a lot of oak wilt, our recommendation is to inject the trees every two years.