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Oak wilt risk status in Minnesota

Current status: Safe

Safe period began November 3, 2015.

Current oak wilt status

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.

High Risk months in Minnesota are typically April, May and June.
Low Risk
months are July, August, September and October.
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.

Thirdinjection 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.

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130 Responses to “Oak wilt risk status in Minnesota”

  1. Kim says:

    I trimmed a few very small, (long enough to have old leaves on them, but small enough to be snipped off with a 1.5″ lopper) branches off one of my favorite oaks yesterday. Now I see the risk status switched to high today. I am kind of worried. Should I be worried or am I ok since yesterday was still “safe”? Thanks!

  2. Anna says:

    We have two more trees to take down, some stumps to grind, and many piles of wood to cover which we are taking care of tomorrow. We have been following the updates here and obviously lost track of time. Is the recent status change (today from “safe” to “high risk”) padded or should we hold off cutting those two trees down (one diseased, the other preventive)?

  3. admin says:

    Thanks for your concern Kim. Do you notice any signs of oak wilt in your area? You should be safe, given it seems the branches were small and you didn’t trim many of them. The best approach is to monitor the tree over the next several months, particularly if it a species in the red oak group which are much more susceptible. – Matt

  4. admin says:

    Thanks for your message Anna. We generally don’t recommend cutting any oaks (either felling trees or grinding stumps) during the high risk period. Do you have any flexibility for holding off on these activities until later this year? If oak wilt is in the area, the use of a vibratory plow to limit root grafting may be an appropriate management action (as is mentioned above). -Matt

  5. Kim says:

    Hi, Matt. No signs of oak wilt in our area, and the tree is a white oak. Thank you for the speedy reply!

  6. admin says:

    Thanks for checking in David. Does the wood still have tight bark on it or has the bark begun to loosen? We encourage you not to move the firewood a long distance, particularly during the high risk period. If bark has begun to loosen, consider moving the wood later this summer if it’s a very limited distance. -Matt

  7. David Craigie says:


    Thanks for the reply! I am not moving it off of my property (2.5 acres), just moving it to my winter wood cribs for burning this up coming fall. I will keep the tarp on it until later this summer or early fall where it sits, and will move it then, after the risk period is passed. I plan on covering it when I move it as well, keep it dry, so it wont be uncovered until ready to burn! Thanks for the advice. D-

  8. Nanci Stoddard says:

    We would like to clear brush (mostly buckthorn) out of an area of oak trees (mixed red, white and burr oaks). There are small oak saplings mixed in among the brush. The plan was to cut all the brush and saplings down to ground level and chip the branches. I am wondering whether cutting off the oak saplings at ground level could potentially expose the root system to oak wilt? We are not aware of oak wilt on our property, but have heard it is in the area in Dakota County.

  9. Ryan says:

    I had 2-3 branches trimmed on a red oak on Mar 28th. 1 was sizable because part of the branch was dead. Maybe 10-12 in diameter wound. Should I be concerned or do you sound the alarm with the High Risk before the bugs come out?

  10. DICK GABRISH says:

    I would like to know why the county is cutting oaks on 221st ave. in East Bethe during this high risk period?

    R. J. GABRISH!!

  11. admin says:

    We would like to know, too! Thanks for you attention to this. -Matt

  12. admin says:

    Thanks for your message Ryan. Recently cut wounds on trees generally close within 72 hrs of any pruning activity. So you are likely safe given the earlier pruning that you conducted. Still, it is best to continue to monitor the tree over the next several months, particularly if you notice oak wilt in the area. (See the DNR’s updated map on the extent of oak wilt in MN). -Matt

  13. admin says:

    Thanks for your message Nanci. Have you considered leaving some of the oak seedlings? We certainly encourage the removal of buckthorn, but leaving the oak seedlings may promote some diversity of ages in your area. If you chose to cut the oaks, we would be mostly concerned with opening wounds for infection. Unless there is a known tree infected with oak wilt in your area, the potential for root grafting is unlikely if you are clearing trees aboveground. -Matt

  14. Kent says:

    Is there a growing degree day number to correlate with the activity of picnic beetle activity? If so it would make it easier to forecast when to stop oak pruning…

  15. admin says:

    Hi Kent. Your question is timely on the number of growing days and oak wilt. Researchers at UW-Madison have an ongoing project that is seeking to generate those values and recommendations. Although the majority of their sites are in Wisconsin, there will be some insights to here in MN. We hesitate to postulate a value now given the varying definitions of degree days (particularly in the base temperature used) and the limited research from MN. We will keep you posted! -Matt

  16. Karla says:

    any new research on bur oak blight? Is macro or micro treatment more effective? Does spraying fungicide now have any effect?

  17. admin says:

    Thanks for checking in Karla. I might suggest you take a look at this bur oak blight fact sheet. It recommends fungicide as a potential option in May/June. Hope this helps! -Matt

  18. Rebecca says:

    Hi good morning!
    We had a tree fall down last night and up rooted. This tree has been dead with oak wilt for some time. Can we cut up the tree right away or should we wait?

  19. Alana says:

    I have tried both macro and micro-infusions on one tree I have treated for several years. I know the few resources out there tend to recommend macro but I had good results with micro last year. That’s what I am trying again this year. The tree I am treating is very large, so the amount of liquid needed for an accurate dose of the chemical is very difficult to get into the tree. One particularly wet spring, we failed miserably trying to get all the solution into the tree, and the results were very poor. I am planning to do the micro-injections in the next week or two. In my limited experience, I think the soil and weather conditions being perfect for uptake is much more crucial than the delivery system. I’m hoping for moist (but not water-logged) soil and a warm, sunny day!

  20. admin says:

    Hi Rebecca- It’s best to wait until the Safe period (beginning about November 1) to cut up any oak wilt-infested trees. This will help the lessen the risk of spreading oak wilt and the fungus to any nearby trees. Thanks for inquiring! -Matt

  21. Brian says:

    When will the next oak wilt status update be, we have moved beyond the traditional low risk date which was for many years was July 1.

  22. admin says:

    Thanks Brian. As you likely noticed, we updated the oak wilt risk to low today. ~Matt

  23. Mike says:

    Is it safe to prune or cut dead branches during the low risk period?

  24. Steve P. says:

    We need to cut down some trees to install a new mound septic system. We have scheduled the system install for mid-October. Most of the trees we will cut are Pines, but we do need to cut down one Oak. Will it be safe to cut it down to a stump in early October?

  25. Geoff Tegeler says:

    Hello, I have a 5 acre property in Prescott WI. I had a few oak trees die from what was suspected to be oak wilt 2 yrs ago. I had an arborist come out and his company used a “ditch witch” to cut the roots around the suspected tress that had died.

    Would I know by now if it was successful or could the oak wilt still be spreading underground in the roots?

    Also, I have the little sprouts or “suckers” that sprout around the base of the oaks (usually within 5-8 feet of the tree). Can I cut these down with the lawn mower during high risk season? I had an arborist tell me it was ok to do that, but I’m not sure if he understood my question.

    Thank You!

  26. Steve P. says:

    We are having our new septic system installed now and the backhoe has bumped into and nicked a few oaks. Should we do anything to the wounds?

  27. Emily Dombeck says:

    Mike, 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.

    – Emily

  28. admin says:

    Hi Geoff-It’s difficult to determine the exact location of the oak wilt pathogen underground. Much of that depends on the soil type–root grafts can spread oak wilt farther in sandier compared to more loamy soils. Although it’s been two years, it’s best to continue monitoring the trees for any signs of wilting.

    We generally don’t recommend any cutting of oaks, whether big ones or small ones, in the high risk period. It’s always best to wait until the safe period (beginning about Nov. 1) to cut oaks, particularly in an area when oak wilt has been found in the past. -Matt

  29. steve says:

    Is it safe now [Nov 2] to trim 4 dead branches on my oak tree? The largest one is about 6 inches and the others 2 or 3 inches.

  30. Emily Dombeck says:

    Hi Steve. It is now safe to trim your oak tree. We have updated the oak wilt risk widget to reflect the new safe status as of November 3. Thanks!


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