Oak wilt risk status in Minnesota

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.
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.

The University of Minnesota Extension Forestry team.

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  1. I live in Inver Grove Heights and have a Wilt Oak problem. Does the state have any free programs or offerings to help with care of the disease?

    1. Hi- The MN DNR has an excellent page that explains some strategies for dealing with oak wilt. It is best to contact a pest specialist, arborist, and/or forester to help diagnose the severity of the oak wilt problem. They may suggest a procedure that interrupts the root systems the infected are. A key concern is not to remove the infected trees prior to root graft interruption.

  2. So far oak wilt is not in the LMF Province according to “Oak Wilt in MN”. Is it just a matter of time or are there abiotic or biotic factors that are limiting to its northerly spread?

    1. Hi John-Thanks for your comment. You’re correct that oak wilt has historically been found in the southern half of the state. However, MN DNR oak wilt in 2012 in the St. Croix State Park (see page 36 our their forest health report). This is very much on the edge of the LMF province in Pine County. As we mention above, the spread of oak wilt is a tremendously localized concerned, even attacking individual trees and pockets of that show no symptoms of stress.

  3. I have a very large red oak that had 4 trunks that came together at the ground. Over the past several years, each of the trunks have died and now all 4 are dead with no leaves developing in the Spring. This is in a woods setting where there are other oaks, both red and white, in the area. Question: If I cut down the dead trunks, would the remaining “liquid” in the dead tree stump go down the roots and be more likely to damage other trees that the roots may be touching of grafted together or has that already taken place. In other words, should I take all the dead down or leave it stand?

  4. I am a little confused, on the fourth recommendation

    “Unless the bark of oak wilt-killed oaks has been removed, that firewood needs to be used on site”

    Isn’t it possible for spores and fungus to cling onto the firewood even if the bark is removed and transporting it could spread the disease.
    There is little information on what removing the bark accomplished except to speed the drying of the firewood. Shouldn’t the suspect wood go through the summer to help age it? We have some people who believe taking the bark off makes an unsafe log ok. Please advise as the harvesting of firewood is such a big issue.

    1. Thanks for your question Ric. The spore mats on red oaks are produced in the spring between the wood and bark and could survive for a period of time, particularly during the high risk period. Regardless of whether or not it is debarked, it is always safest to burn that infected wood on site. -Matt

    2. Thanks for your note Frances. Did you notice leaves wilting from the top of the tree down and/or discoloration from the leaf tip to the base? Those symptoms would certainly indicate a problem with oak wilt. Otherwise, early leaf fall can occur if a heavy seed year has occurred, or if there may have been recent activity near your oak (e.g., construction, soil compaction). Best to continue monitoring the tree into next year. -Matt

  5. There are many red oaks, both large and small, on my property. The trees are dense enough so that an infection in one tree could theoretically spread via root grafts to every tree. I am considering strategically cutting down some trees in order to create groups of trees with gaps between that are large enough to eliminate root grafts between these groups of trees. Will this work? Will the roots from a removed tree still act as a conduit for the fungus? If so, should I treat the cut stump with a chemical to kill the roots? Or, will that chemical spread through root grafts and kill nearby trees?

    1. Thanks for your question Greg. It doesn’t sound like you’ve noticed any oak wilt infections in your area? If you have, methods like vibratory plowing may be your best action to prevent its spread. If oak wilt hasn’t yet made it to your woodland, then it’s good you’re thinking about making your stand resilient to any future infections! It’s difficult to get a sense of what size gaps you’re thinking of and the proximity of other infections in the area. It’s best to consult a forest health specialist or to have a trained professional look at your specific stand of oaks. -Matt

  6. We live in White Bear Lake. We found a supplier of firewood from Stacey, who has a lot of oak available because his trees died of oak wilt. He said it is several years old, cut and dry.

    does that wood present a threat to our oaks if we bring it home and store it in an outside woodpile?


  7. Hi Kathryn- We generally do not recommend transporting any firewood off site from infected trees with oak wilt. If the firewood is indeed a few years old and particularly if the bark is removed, then there is much lower probability that nitidulid beetles are present. If you plan on storing it at your home, we recommend placing the wood beneath a tarp as per our fourth recommendation above. -Matt

  8. My wife and I live in North Oaks and have quite a few oak trees on our property. Based on your recommendations, we just did some minor trimming of some of these trees this past weekend (March 8, 2015). We know this is generally considered the safe time to prune, but we are concerned about the unusually warm weather that we are now experiencing (50-60s). Should we be worried? Should we be sealing the wounds with shellac? Please advise. Our oaks are very old and precious and we’d hate to be jeopardizing them. Thanks!

  9. I know that March is considered “Safe”, but with a week’s worth of temps in the 40’s and 50’s, with another week or more of similar temps, does that move March out of the safe zone? I was told that to be truly safe, temps need to be below freezing.

    1. Thanks for your messages Ethan and Steve. As of today (March 13), we are still in the safe period for oak wilt. Fungal pathogens and insect vectors are not yet active, but will be very soon. Keep checking our site and we’ll announce when the status in oak wilt changes. -Matt

  10. Thanks Matt. A few follow-up questions (technical in nature): When we prune our oaks in the safe period, what makes it safe? Is it because the wounds do not produce sap or an attractive odor? I understand that the fungus/insects are not active, but what happens to the wounds when it begins to warm up? And why are the insects and fungal pathogens not a threat any longer? Is there a preferred period of time between the act of pruning and a warm-up/appearance of insects/fungus? Just curious, because-—when I look at the wounds—they look open and vulnerable.

    1. Hi Steve- we generally consider a period as “safe” based on whether or not the sap-feeding beetles are active or not. The beetles become active after an accumulation of degree days in the spring where they can transit the fungus. While we generally recommend shellac on any open wounds IMMEDIATELY following pruning (if it’s done during the low risk period), you might think about applying shellac on the pruned trees. But being in the safe period, you should still be safe. -Matt

    2. Thanks Matt. To simplify my question, I am wondering why—once the beetles become active in April-June—they are not able to transit the fungus to the wounds made earlier during the safe period? I am assuming that oak trees are only vulnerable for a short time after sustaining a wound and that they begin to heal over almost immediately–even in their dormant stage.

    3. Exactly Steve. The beetles are attractive to fresh wounds found on oak trees, generally any wounds that have occurred within the past 72 hours. Most wounds that have existed for more than three days, although fresh to the human eye, are unattractive to the beetles that transmit oak wilt. Wounds can heal rapidly on trees, which in the dormant season, is reflective of their ability to use reserves from tree storage. It is the xylem transport (the flow of water through a tree) which the beetles can transmit oak wilt. Thanks for following up! – Matt

  11. And to follow up with Steve P’s question, are stumps that I’m creating now by cutting down infected dead trees vulnerable to infection? Do I need to treat them? Can I grind them without worry that the fungus can travel through the stump and infect healthy roots from other trees?

    1. Thanks for you comment Todd. Recently cut wounds on trees and their associated stumps generally close within 72 hrs of any activity (as we mention in the previous comment). Any stumps created during the risk period are of high susceptibility to oak wilt. We generally discourage any maintenance of oaks during the risk periods, including by grinding recently-cut trees. -Matt

  12. My tree guys were scheduled to come out Tuesday but could not get out until today. I checked the website and it said SAFE so I let them go ahead and trim dead limbs from seven of our mature oaks. With yet another day of warm weather, I’m concerned.

    A) How long does it take for fresh wounds to become low or no risk?

    B) How often does the site here get updated?

    Thanks for supporting fellow tree lovers with this info. I’ve already shared links to this site with a bunch of friends.


    1. Thanks for sharing our site with others Steve! We greatly appreciate it. Yes, we are still in the safe period for oak wilt (as I write this on March 23, 2015). As more warm weather begins to take shape this spring, we are talking closely with our forest pathologists and will update the page to HIGH risk when appropriate. We generally say April 1 is the target date for this change, but it’s generally within plus or minus a few weeks of that date. Generally the beetles will only be attracted to fresh wounds within the first 72 hours of a wound, so pruning last week is a near zero risk. -Matt

  13. I’ve been cutting down red and white oaks during this safe period, but at some point I want to get the stumps ground down. Is it possible for the stump grinder to transmit wilt by previous contact wth an infected tree? Also I would think the stumps are still alive so would grinding them later this spring pose a risk to other trees? Thanks.

    1. Thanks for checking in on this Mark. It’s unlikely that the sap-feeding beetles can transfer from the equipment being used for stump grinding your trees. However, any fresh wounds to oaks are attractive to the beetles during the risk periods for oak wilt. We generally don’t recommend creating any wounds to oaks during the risk periods, either by pruning or stump grinding recently cut trees. It’s better to avoid any maintenance to oak stumps during the oak wilt risk period. -Matt

  14. I have followed your advice you gave me last spring (comment above under same name) and my oak wilt trees were split, and covered in a day per your recommendations. It is approaching the one year time line and my new questions are as follows: Should I continue to keep the poly tarp on it, or can it be removed now? I want to use the wood for home heating this fall, and I want to move it closer to my home, should it be covered after I move it? Is it safe to burn this fall? This July will be two years since the trees died, is it safe enough to grind the stumps this summer? Thank you for your help!

    1. 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

  15. 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!

    1. 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

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

  16. 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)?

    1. 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

  17. Matt,

    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-

  18. 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.

    1. 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

  19. 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?

    1. 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

  20. 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!!

  21. 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…

    1. 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

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

  23. 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?

    1. 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

  24. Karla,
    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!

  25. 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.

    1. 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

  26. 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?

  27. 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!

    1. 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

  28. 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?

  29. 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.

    1. 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!