Q. What is oak wilt?
Oak wilt is a fungal disease that kills thousands of oak trees every year. Oak wilt spreads in two ways: Through root grafts between similar species, radiating outward from a central infected tree; and over land, carried by oak sap beetles carrying fungal spores from tree to tree.
Q. What are the “risk season” references?
A. There are three risk season timeframes: High Risk, Low Risk and Safe. They refer to the probability that oak wilt will infect a tree. Specific dates for timeframes vary depending on weather conditions.
High Risk months in Minnesota are typically April, May and June.
Low Risk months are March, July, August, September and October.
Safe months are November, December, January, February and March.
Q. What are these probabilities or “risks” based on?
A. Three criteria are considered. First, is the fungus that actually causes the disease active? Second, is the beetle that carries the fungus to the oak active? Third, is there oak wilt in the area? If all three criteria are met, then the transmission of oak wilt from one area to another is very likely. This is referred to as “over-land transmission” of oak wilt.
Q. What are the best ways to either avoid or minimize the probability of oak wilt infection?
A. First, avoid any wounding during the High Risk (and hopefully, the Low Risk) period…no pruning, no construction activities near the oaks. If a tree is wounded, seal the wound quickly (within 15 minutes) with one coat of shellac (preferable) or a water-based paint. If oak wilt is in the area, it’s the High Risk season, and the wounding is unattended for more than 15 minutes, the probability of infection rises dramatically.
Second, and especially important if oak wilt is established in an oak woodland, prevent the spread of the pathogen through root grafts by cutting through the connecting roots using a vibratory plow. This will need to be done by a professional, preferably a Certified Arborist and if done correctly is a very reliable technique to reduce the amount of oak wilt spread.
Third, injection of a chemical fungicide may reduce the risk of oak wilt-related tree mortality from root graft infections for 2-3 years. If combined with vibratory plowing, chemical treatment may provide long-term protection. This should only be done by a trusted and experienced professional that is licensed to apply pesticides and ideally is a Certified Arborist.
Fourth, do not move firewood from oaks that have died from oak wilt off of or on to the property in question. The red oak group in particular harbors the fungus for several months under their bark, even if they’ve been cut down. Unless the bark of oak wilt-killed oaks has been removed, that firewood needs to be used on site (burn before the next High Risk period) or covered completely. If the wood with the bark on is tarped, the tarp must be at least 4 mil. thick and preferably clear in color. The tarp should be weighted down at the ground line and sealed with soil at the ground line so no beetles can crawl in and out. Keep the wood covered for at least one full year after the tree has died.
Q. Can any beetle move the fungus from one area with oak wilt to another?
A. The nitidulid beetles that move oak wilt are commonly called “sap-feeding beetles.” There are only a couple of these types in Minnesota and they’re very small.
Q. Can an oak become infected during the Low Risk or Safe periods in Minnesota?
A. Oaks can become infected during the Low Risk period, but the probability is very low. However, since it could happen, it’s best to delay pruning of the oaks until the Safe period or to quickly seal the pruning wounds with shellac or paint to avoid attracting the beetle if pruning during the Low Risk period is unavoidable. During the Safe period, there is virtually no risk that an oak can become infected with oak wilt by over-land transmission of the fungus.
Q. Is “over-land” transmission the only way oaks can become infected with oak wilt?
A. No. Most oak wilt is spread via root grafts. Oaks of similar species, for instance red oaks, can root graft with other oaks nearby…easily within 60-80 feet of mature oaks. When this happens, fluids can pass from one oak to another, including fluids that carry the fungal pathogen. Oak wilt spreads from one area to another (distances greater than a quarter mile or more) via the beetles carrying the fungus. Once the disease is established in a tree, it spreads from that tree to others via root grafts.
Q. Are all oaks affected the same way?
A. The red oak group (red, black, Eastern pin, northern pin and scarlet) are more seriously affected by the disease-causing pathogen. Once infected, they do not recover and die very quickly, often within 4-6 weeks of infection during the growing season. The white oak group (bur, white, bicolor) can become infected, but they often live with the disease for a long time before dying. This lengthy period allows tree care professionals to intervene, even after infection, and can often save the trees.
Q. Once the tree becomes infected, is there any treatment?
A. For oaks in the red oak group, no. For oaks in the white oak group, yes. A qualified tree care professional will prune out the dead wood (if the disease hasn’t progressed too far) and if licensed, inject the tree with a systemic fungicide. In most cases, the trees will recover if there are no other health problems affecting them.
Q. In areas where oak wilt has killed the oaks, should replacements be other than oaks?
A. Genetic diversity is always a good way to make a forest, woodland or landscape healthier. Few insects or disease-causing pathogens kill wide varieties of trees. If the area that suffered oak wilt losses is dominated by oaks, replant with other species such as sugar maples, black cherries, hackberries, white or river birches or maybe some of the disease-resistant American elms. If oaks didn’t dominate the landscape (made up less than 10% of the tree population), some of the replacements can be oaks, especially those in the white oak group.
Q. Is there any other resource that can provide more detailed information and pictures of oak wilt?
A. The publication “Oak Wilt in Minnesota” by David French and Jennifer Juzwik is probably the best available resource. Additionally, the Minnesota Department of Natural Resources web site has very valuable oak wilt information, and the University of Wisconsin Extension has an excellent short publication called Oak Wilt Management: What Are the Options?
Q. Can I add the University of Minnesota oak wilt widget to my site?
A. Yes! You can download the embed code here.