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Oak wilt risk status in Minnesota

Current status: Safe

Safe period began November 1, 2014.

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 40 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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86 Responses to “Oak wilt risk status in Minnesota”

  1. Ryan Blaedow, MNDNR says:

    There is no risk of spreading the disease by cutting wood from a tree that has been killed by oak wilt. I would only caution you to sterilize your saw before cutting/pruning healthy oaks afterwards becuase it is possible to spread the fungus on contaminated tools. Rubbing alcohol in a spray bottle would be just fine for this purpose. If the wood has been dead for more than a year, there is little risk of spreading oak wilt if you transport the firewood. Be aware however that movement of hardwood firewood is restricted out of Hennepin, Ramsey, Winona, and Houston counties because of Emerald Ash Borer quarantines. Also, while the risk of transporting oak wilt on aged firewood is very low, we always recommend utilizing local firewood if possible to reduce the risk of spreading other insects and diseases.

  2. susan hopp says:

    We live in Mpls and have a cabin in Amery WI. We have 4 oaks that have dead branches directly above our deck and I have been very concerned wth safety as we sit there. I now have a tree trimmer lined up to trim only the dead branches. He only does residential on weekends and it has snowed the last two weekends! Since he climbs the trees, needless to say, it’s been too dangerous. He is planning on this Saturday morning. I am watching the temps and am concerned that we will be in the high risk time by then. He promises to get there by 7 am before it warms up and has agreed to spray the larger limbs asap. (Why not the smaller ones, I need to ask him!) What do YOU recommend? If the branches are dead, is the risk still high? Help! Thank you. S

  3. Kathryn Sartain says:

    We just had a large red oak split by the wind last night. A large portion fell on our second garage. Today we have had it cut up and pulled off the roof, but the section that is still standing is splintered on one side. Since the date is June 22, I assume we are still in high risk time. There was no way to seal the tree during the night and storm, or even now because of the splintering 12′ up. There was no wilt in this tree or on our own property now. We had plowing done years ago. There is still some further away in the neighborhood (Anoka County). Is the wood from this tree at risk of attracting the bugs and is there any way it would be a danger to our other trees? We will probably have to remove the remaining part of the tree since there is split down the middle of the trunk. Should we be concerned about whether infection could start and go into the roots? Would stump grinding stop it?

  4. Krysta says:

    After this huge storm last Friday, I saw some pieces of bark on the ground. A couple and not more than a foot in length. I looked for where so I could seal it (though even if I had, it would’ve been too late by the time I saw it) but couldn’t f ind it.

    Now what do I do. This is a gorgeous 200+ year oak. I don’t know which kind it is. Do I try to do a preventative fungicide just in case?

  5. admin admin says:

    Hi Krysta. I would not worry about trying to seal up the areas that lost their bark. Anything that was going to get in to the tree through the new wounds will be in there by now. As for whether or not the fungicide treatment would be advised, that is up to you based on how important the tree is to you. Based on the article linked above addressing the fungicide issue, a propaconizole treatment would be expected to protect the tree for 2-3 years, after which another treatment would be needed. As the fungus does seem to remain alive in the roots of the tree, treatments would be needed every few years if the tree is indeed infected.

  6. Rich says:

    I had a red oak die in the fall of 2011. I thought it was oak wilt but was told it wasn’t. Summer of 2012 that tree was taken down. This week a tree about 20 feet from that one has dropped most of it’s leaves I’m sure it’s oak wilt, and have three more large red oaks within 20, 30 and 40 feet, is it to late to cut the roots on these others?

  7. Brian Weidenfeller says:

    High risk status August 1st, this must be typo.

  8. admin admin says:

    Hi Rich. It’s generally recommended to create primary and secondary barriers (root trench treatments) not only immediately around infected trees, but also on the far side of nearby asymptomatic trees. A few resources that explain this in more detail, including graphics with recommended trench locations: How to identify, prevent, and control oak wilt (USFS), Oak wilt in Minnesota (UMN Extension), and Oak wilt: Identification and management (Iowa State University). Hope this helps.

  9. Angie hanatsd says:

    We had oak wilt awhile back and we injected our trees, and a year later the treatment didn’t work, so down came 13 large oaks. The cost was several thousands of dollars. My husband pointed out to me -looks like its come back, so far three of our effected we have about 9more to go. Is there any tax breaks of having them injected again? What’s one to do on a fixed budget? I am so tired and very disappointed that its come back!

  10. admin admin says:

    Hi Angie. Very sorry to hear about your trees. It sounds like the oak wilt disease must have spread beyond your control zone. You mentioned treating some trees and removing them later, but I hope that you also hired a company with a vibratory plow to sever root grafts. Underground transmission among trees through root grafts is a very common way for an established oak wilt infection to spread. You can read more about oak wilt control options at the links near the end of the Q&A content above. I hope this helps and am sorry to hear of the loss of your trees. Oak wilt can indeed be expensive and difficult to control.

  11. David Craigie says:


    I just had a red oak and a white oak taken down on the 31st of March that had died over the summer from oak wilt. The Red Oak was a specimen that was over 70 years old (I counted the rings) and I’m sorry to see it go. The white oak was 16″ diameter, and was only 10 feet from the red oak. I live in a mostly wooded area, and I’m worried about infecting other trees. Some of the logs that were cut up are around 300 lbs. each, and I’m going to have to split them before I move them. I want to tent the wood as well, so I can use it for heat in 2015. How long do I have to split the wood before I run the risk of infecting other trees? Can I split it at my leisure, or do I need to move on it? Thank you!

  12. Gina Hugo says:

    Do you know of any data on spring spore mat formation related to growing degree days? Going by the calendar we have entered the high risk period – however we have had less than 10 growing degree days. I look forward to some feedback.

  13. admin admin says:

    Hi Gina. The major factor driving our risk status season boundaries is not mat formation, but the flight activity of the sap beetles that carry the oak wilt fungus, primarily the beetle Colopterus truncatus. We are fortunate to have one of the foremost experts on oak wilt distribution, Jennifer Juzwik of the US Forest Service, stationed in St Paul. Dr. Juzwik has authored a number of studies of oak wilt distribution by nitidulid beetles, and our risk seasons are based on her analysis.

  14. admin admin says:

    Hi David. Sorry about the loss of your trees. Your question is best answered in the fourth item under the question “What are the best ways to either avoid or minimize the probability of oak wilt infection?” above. Basically, one way or another you will need to tarp those logs pretty quickly — we recommend before the start of the next high risk season, which is imminent. You could tarp them where they are or split and move them, but we recommend keeping the red oak tarped for at least a year. There is a bit more info above.

  15. Vidar Romarheim says:

    I have a theory based on experience on my property. Oak wilt will not necessarily spread to trees next to it if you wait two years after death to cut it down. Anyone else have any experience with this?

  16. Tom says:

    Connexus Energy is trimming trees in Blaine, MN, including oaks. I inquired and they indicated they go by your Oak Wilt Risk Status, which indicates High risk will begin Monday, April 21, 2014. It seems unwise to me for them to be wounding many oaks through pruning just days away from the High Risk period. Are they justified in this pruning of oaks right now?

  17. admin admin says:

    Hi Tom. Good question. We do plan our risk periods with situations like this in mind, and what the utility company is doing is not inconsistent with our risk management recommendations. While there is some risk associated with pushing any ecological threshold to its limit, that risk has to be balanced by practical considerations. I would not be overly concerned in this case, although I can understand your concern.

  18. John says:

    I am wondering about removal of live red and white oaks that are not affected by the oak wilt during this period? Is it possible to do or to wait until the high risk is over?

  19. John Hedstrom says:

    I had storm damage on about 50 oaks on our property last June. Most we’re broken off and were impossible to seal. We have oak wilt on the property and some trenching has been done. I’ve gotten about a quarter of the hangers on the ground. Can I cut the stuff on the ground during the high risk period? Do I need to take down infected trees as soon as they are identified? Am I likely to loose all the wounded trees?

  20. admin admin says:

    Hi Vidar. That theory is contradicted by overwhelming evidence that oak wilt will indeed spread if left untreated. Oak wilt is less virulent on members of the white oak group (bur, swamp white, white, and other oak species), and if those are present on your property that may explain your observations.

  21. Ron says:

    I have five small oaks (white I think) that I planted as sticks about 13-15 years ago. A couple are about 8 feet tall and a couple are closer to 20 feet tall. They are only 2-4 inches in diameter at the bases. Three years ago we notice early turning of leaves at the very top on one or two. Second year a little worse and I realized it was Oak Wilt. I cleaned up fall leaves pretty well and the third year it was still present but did not affect nearly as large a portion of the trees. (it was at worst 1/3rd affected in year 2 and less than that in year 3). I would like to be more aggressive this year if you think that there is anything I can do to save small trees like this that won’t cost an arm and a leg. Your suggestions would be greatly appreciated. Thanks, Ron

  22. Sue Gagas says:

    When is it safe to grind the stumps of infected trees? We cut them down 2 years ago.

  23. admin admin says:

    Hi Sue-The oak wilt fungus is attracted to the scent of freshly cut oak. It’s uncertain how fresh wood can be after 2 years, so it’s safe to wait until the late summer/early fall for grinding oak stumps.

  24. Houa says:

    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?

  25. admin admin says:

    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.

  26. John says:

    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?

  27. admin admin says:

    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.

  28. Gordon says:

    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?

  29. Ric asks: says:

    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.

  30. admin admin says:

    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

  31. Frances Tierney says:

    Almost all the leaves are down from my Oak Tree. Is that normal or something I should worry about.

  32. admin admin says:

    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

  33. Greg J says:

    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?

  34. admin admin says:

    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

  35. Kathryn says:

    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?


  36. admin admin says:

    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

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