Oak Wilt

 

Helpful publications:

Whats in that woodpile- University of Minnesota Firewood ID (.pdf)

Oak Wilt (USFS)

Oak Wilt in Minnesota

How to Identify, Prevent and Control Oak Wilt

Frequently Ask Questions

Which oaks are susceptible to oak wilt?
All oaks found in Minnesota are susceptible to oak wilt. There are some differences between the rate of disease development and ability to produce oak wilt mats.

• Oaks from the red oak group (includes black, northern pin, eastern pin and northern red oaks) are most susceptible to oak wilt due to physiological and anatomical differences from the white oak group. Once infected, red oaks die within a few weeks to months (usually within the same season). Red oaks also commonly produce oak wilt fungal mats, fruiting bodies which produce spores and spread oak wilt using insect vectors.

• Oaks from the white oak group (includes bur, white and bicolor or swamp white oak) are less susceptible to oak wilt. After infection, mortality may take 5 – 10 years. Spore mats are uncommon on white oaks.

 What is meant by high risk, low risk and no risk period?
The different risk periods are defined by two factors: the presence of oak wilt-carrying beetles and of oak wilt fungus mats.  Beetles have to be active to spread to the oak wilt spores. The fungal mats are formed most commonly when temperatures range from cool to warm (43 to 85 F) and when the sapwood moisture content of recently killed oaks is optimal for the fungus (these conditions most frequently on red oaks that were infected and killed the previous season).  Peak mat production and flight activities for key insect vector species often occur simultaneously in the spring in the Upper Midwest, hence the high risk period during April, May and June.

For more information on oak wilt transmission during the low risk period, see:
Low risk of oak wilt pathogen spread by sap beetles during summer and fall

 

What are the symptoms?
In the red oak group, symptoms appear in late June and early July (a few weeks after infection). Leaves in the upper portion of the canopy turn an off-color of green. The symptoms progress to the lower parts of the canopy and the leaves begin to turn bronze at the leaf margins.  
In the white oak group, symptoms are more subtle. Individual branches die slowly every year. Leaf symptoms, albeit slower, are similar to red oaks. Cross-sections of branches or the trunk reveal a series dark brown rings or spots

Images of oak wilt provided by The University of Minnesota Extension www.extension.umn.edu/distribution/naturalresources/DD3174.html

Left: Read Oak leaves in process of wilting. Right: Cross-section of White Oak branches show discoloration often seen as small dark dots in the wood just under the bark. Figure 4. A sprulating mat produced during May by the oak wilt fungus between the bark and wook of a tree that wilted during the previous summer. The dark area in the cener of the mat are pressure pads that crack open the bark.

 

Are there any look-alike symptoms?
Yes, oak anthracnose, two-lined chestnut borer and drought produce symptoms that are often mistaken for oak wilt. Consult a certified arborist or community forester to help with identification.

What are my control options?
Your first line of defense is keeping your trees healthy. Don’t prune or wound your tree during the high risk period, monitor your oaks throughout the season and clean up storm damage in a timely fashion. If a wound does occur, cover it with paint within a few minutes.
If you find that your woodlot has oak wilt, it is important to devise a plan before reaching for the chainsaw. Although, the eventual goal is to remove or treat infected trees, cutting a tree may accelerate the movement of the fungus into grafted roots systems. Survey all oaks within in 50 ft (roots may spread further in sandy soil) of any infected trees. Most root grafts occur between oaks of the same group (i.e., red oak group), however, although less common, grafting does occur between bur oaks and red oaks.
Infected trees should be mechanically isolated from other trees. In Minnesota, a vibratory plow is commonly used to sever root connections between healthy and infected trees. After root severing, infected red oaks should be cut and properly utilized (e.g. cut into firewood length logs and covered with plastic).
Infected trees from the white oak group, with less then 30% of the canopy affected, can be treated with propiconazole (the most widely used trade names are Alamo and Honor Guard PPZ Ò). Treatment is done every two to three years when disease pressure is high. Preventive treatment of healthy oaks within root-grafting distance of infected trees may prevent disease development for two or three years. Examine and consider treatment of all oaks within the root grafting “network”; root grafts can transport the fungus from tree to tree throughout a woodlot. On some occasions, the fungus can “leap-frog” through the root system of a treated tree to adjacent root-grafted trees.

I think I may have oak wilt, how can I be sure?
Consult a certified arborist or licensed tree inspector to positively identify oak wilt. Alternatively, send a sample to the University of Minnesota Plant Disease Clinic. Call the clinic at 612-625-1275 for sampling instructions.

Is there financial assistance available for oak wilt control or tree removal?
Many communities have cost-share funding available to manage oak wilt. Contact your city or community forester for more information. In rural areas, consult your county or DNR regional forester. You may also contact the Department of Forest Resources Extension and Outreach office for help in finding your respective professional.

Will newly replanted oaks be more likely to get oak wilt?
In general, replanting of oaks in an old oak wilt disease center does not result in disease occurrence in the replanted trees.  Root grafts are not thought to form with the dead, diseased oaks.  Thus, it is only through insect spread from active oak wilt centers in the vicinity that disease would occur in replanted oaks.  Oak trees that are healthy are not susceptible to oak wilt in an infected area. Soil, chipped and debarked wood will not harbor the oak wilt fungus.

Where can I go for help?
Consult the University of Minnesota Extension publication “Oak wilt in Minnesota” or the US Forest Service publication “How to identify, prevent and control oak wilt” for more detailed information. For field assistance consult an ISA- certified arborist, certified forester or licensed tree inspector. A list of certified arborists can be obtained at: http://www.isa-arbor.com/findArborist/findarborist.aspx. The Department of Forest Resource Extension and Outreach office staff is available via phone (612-624-3020), email (treeinfo@umn.edu) or in-person (116 Green Hall, St. Paul Campus).