Oak Wilt in Minnesota

Comprehensive overview of the fatal fungal disease

Even though the origin of the pathogen that causes oak wilt isn’t known, it was first detected in 1942 in Wisconsin. Here in Minnesota, it was first discovered shortly after some time in 1945. Oak wilt is a fatal disease that affects all species of oak trees across the state, more susceptible is the red oak group, the white oaks are less susceptible and show much slower decline. The fungal disease spreads to healthy trees below ground via connecting roots or above ground by insects, we as humans play a vital role in the spread of the disease by transferring infected wood from infected areas to new regions.

The fungus, Bretziella fagacearum previously known as Ceratocystis fagacearum, will invade the vascular system of oaks, leading to death of the infected tree. While cutting off the water and nutrients from traveling throughout the tree, you will start to see leaf discoloration, wilting of leaves, leaf dropping, leading ultimately to the death of the tree. On mainly the red oaks, you may see what are called oak wilt fungal mats, these are typically dark brown to black in color. The chemical produced by these mats will attract sap beetles that will then transmit the disease from tree to tree. 

As we take a deeper look into the fatal disease, it’s crucial to remember to avoid touching your oak trees from April through July or beyond, to avoid the spread. While there is no complete cure for oak wilt, you may be able to prevent oak wilt with injections, these should only be preformed by a qualified arborist. More harm than good can happen if done incorrectly. Once a tree has shown signs of oak wilt, it is too late, injections will not cure a tree with oak wilt. There is an exception, if a species from the white oak group is showing symptoms but less than 30% of overall crown is showing signs, injections could possible reduce and slow the progression in that tree, it could stop it all together, but there is no guarantees with these rather expensive injections.

Oak Wilt Mat 001
Fungal mat spores (Fig. 1) Photo by Dr. Tara L. Bal - Michigan Technological University

VISUAL SYMPTOMS & SIGNS OF OAK WILT

Oak wilt can attack all species of oak trees, the most susceptible to this fungal disease is the red oak group. Minnesota has only three species from this group: northern red oak, northern pin oak, and black oak which can be found in the extreme southeastern part of the state. The white oak group, which is less susceptible to oak wilt but still can be fatal, has four species here in Minnesota: white oak, bur oak, swamp white oak, and the chinkapin oak. Each year hundreds of thousands of oak trees are killed due to oak wilt disease, a disease that has no known cure to eliminate once a tree has started to show symptoms and signs. Symptoms in each group can slightly vary but with a few different identification factors, you may be able to distinguish if your oak tree has symptoms of the disease. Listed below are the more common symptoms to spot in each type of oak trees.

Red Oak Group – northern red oak, northern pin oak, and black oak

Leaf Symptoms

Leaves will have brown edges and a green center; they will continue to brown towards the center and base.

Wilted leaves drop off quickly; leaves will be on the ground around the tree during spring and summer. Typically, red oaks will lose 90% of their leaves in one to two months.

Wilting will typically start at the top or outer parts of the crown and will make its way downward rapidly.

Bark & Limb Symptoms

If bark is peeled back on a limb with wilting leaves you could see a bluish staining on the wood.

Cracking bark or oak wilt fungal mats may be visible, about half of the red oaks infected with oak wilt will produce a fungal mat.

Tree Mortality

Oak wilt progresses rapidly in red oak species, trees do not survive oak wilt.

Oak wilt infected trees by root grafting, trees will usually wilt out and lead to the death of the tree shortly after leaf-out of the following year.

Due to the bright red-brown foliage or extremely bare appearance, you can easily spot these red oaks that are infected with oak wilt.

White Oak Group – white oak, bur oak, swamp white oak, chinkapin oak

Leaf Symptoms

Similar to the red oaks the white oaks leaves will typically have brown edges and a green center, working its way towards the center and base of the leaves, but symptoms may vary.

The wilted leaves will drop off quickly and will be scattered around the tree.

Symptoms will initially appear in one or several scattered limbs but may not progress and develop further signs until the following years.

Bark & Limb Symptoms

Dark brown to black discoloration in the wood may be visible if there is peeled back bark on limb with wilting leaves.

Ring like or narrow discolored wood may be visible in the cross section of an infected limb.

Tree Mortality

Oak wilt progresses very slowly in white oaks compared to red oaks.

While there may be visible signs of oak wilt one year, it could take years before you see signs of oak wilt in other sections of the tree. In some instances, it can take upwards of 20 years for certain white oaks to die completely from oak wilt.

Although death appears slower and sometimes isn’t noticeable immediately, usually when it showing signs of oak wilt, it is typically too late to save that tree.

HOW DOES OAK WILT SPREAD?

Oak wilt is spread in two ways, either insects or by their root systems. Technically speaking, there is three ways the disease spreads, the third being human interaction. Transporting infected wood and storing firewood of an infested oak tree increases the spread drastically. These insects are attracted to the chemical aromatic smell produced in the fungal mats, then they will transmit the fungal disease, and carry it to new areas which then become new centers for oak wilt infection zones. Once an area has started to have oak wilt death, it will then spread tree to tree through the roots of those trees that have grafted together. In areas around the state with deep, sandy soils and closely growing oaks the spread can happen rapidly.

Colopterus truncatus and Carpophilus sayi, two types of sap beetles, are the predominant transmitters of oak wilt here in Minnesota. While in other areas across the country oak bark beetles play a vital role in the spread, they do not affect us here in Minnesota. The fungal mats that attract these sap beetles are usually seen in the produced in the spring and fall. The mats produced during the spring, April through July, are the ones that are most important in the disease spread. That is why it’s so important to NOT cut down or trim any oaks from April through July, but ultimately you shouldn’t do any sort of trimming until it is into the dormant season, late November through early March is ideally the proper time but weather variables can adjust these dates.

If a tree infected with oak wilt is cut down or trimmed out of ‘season’ there can be some negative impacts to the immediate ecosystem it was once living in. When removing an oak tree during the spring and summer months, even early fall, if there haven’t been any underground preventive measures in place, the oak wilt pathogen can be quickly transmitted into nearby root systems via root grafts. The red oak group will typically graft and transmit the oak wilt fungal disease much more often than the white oak group. Just with anything tree related, there are many factors that can influence the likelihood and distribution of oak wilt from oak to oak; soil makeup, terrain, type of oak species, and the size of the tree itself. Typically, oak trees of the same species within 30’ are at higher risk for becoming a victim to oak wilt, there has been cases where even distances up to 50’ have been reported. While grafting between two trees is more common for trees of the same species, it is possible for two different types of oaks to graft together. This is why underground mitigation efforts should be focused on first before removing the tree itself.

Oak Wilt Transmission (Fig. 2) Illustration by Forest Pathology

To help greatly slow the spread of oak wilt drastically we all should follow these important guidelines for handling infected wood, posted by the MN DNR. Timing is everything, dormant season is ideally the only time we touch our oaks, but that isn’t always possible. Avoid moving firewood or logs during the high-risk period and avoid moving logs and firewood out of the high-risk zone. For an oak tree that has wilted during the growing season it’s best to wait till the fall or winter to fell the tree. If removing in the fall, there should be some underground root work done to eliminate spread via root grafts. To reduce removal costs and to follow guidelines regarding oak wilt as best as possible, oak trees should be removed strictly during the winter months. They can then be treated on site or transported to an approved waste site for oak wilt. Treatment options on site are debarking logs or cutting wood into fire wood lengths and stacking to let dry.

If a tree must be removed during the high-risk period for oaks, the wood should be destroyed via chipper or burning of materials right on site (chips will not spread oak wilt, so they can remain on site if requested), sometimes that can’t be achieved due to various reasons. If a tree must be removed during the high-risk period, proper precautions should be followed, MN DNR has proper guidelines listed here. For areas where oak wilt is common, infected trees add minimal risk to the area, only where local municipalities don’t require dead trees to be removed immediately, it’s a great option to leave the tree standing for at least one year after death, the tree will no longer be able to produce the fungal mats.

OAK WILT MANAGEMENT & MITIGATION

Oak Wilt Root Cutting Plan (Fig. 3)

There is no way to completely eradicate oak wilt disease, managing oak wilt can be achievable but a few key things play a deciding factor. On a residential city property where you may only have a few oak trees, you may be able to save trees if only one is showing signs of oak wilt. On a property with an abundance of oak trees, it will be a tougher task but not completely impossible to do, although if a forest is too far infected with oak wilt, there may be no saving efforts possible. With that said, it is important to act swiftly and promptly but only after the appropriate plan for the circumstances has been thought out by a professional.

The biggest and most important thing when it comes to managing and mitigating oak wilt is to NOT trim or prune during the warmer months, especially the early part of the warm season, April through July. If a tree has wilted and died from oak wilt during the growing season precautionary steps should be taken to avoid spreading the disease further. First thing, the tree should remain standing unless it poses a serious hazard on the property where it stands. If no immediate hazard is present then there are two options for removal, one is that the tree can be felled in the fall if root cutting takes place, otherwise the tree should be felled in the winter and properly disposed.

Root cutting and other measures of underground mitigation should only be performed by a trained professional, while it may seem like a straightforward task, there can be some negative impacts if done incorrectly. In attempts to save larger quantities of oak trees on a property, costs can be rather expensive due to the equipment and appropriate planning required. It is important to note that during this process there is potential for a handful of oak trees to be ‘sacrificed’ in order to preserve the rest of the forested area. 

Root cutting can be a great way to vastly reduce the spread of oak wilt in a abundant area, especially if caught earlier enough. While root cutting can be the most effective method in reducing oak wilt, it must be known that no treatment will guarantee that an oak tree will not contract oak wilt during its lifespan.

Contact a professional to start preserving your oaks today. Remember that time is of most importance with oak wilt!

Oak Wilt Root Cutting (Fig. 4) Photo by Bill Cook, Forester & Biologist - MSU Extension

Since first being discovered in 1945 oak wilt has been spreading from the southeast into east-central Minnesota and still continuing to work its way northward starting to encroach on northern forests. As of March of 2024, oak wilt has infected trees in over 40 counties in Minnesota, although the greatest concentration of oak wilt currently is in; Sherburne, Anoka, Isanti, and northwestern Dakota counties. 

The MN DNR updates a yearly map to track the spread of oak wilt, for the most up to date map check out the DNR website here

Additionally, to help mitigate the spread of oak wilt, the MN DNR is asking residents in the following areas to report any known or highly suspected oak wilt; Kanabec County, northern Mille Lacs County, northern Morrison County, northern Pine County, central Stearns County, and all the areas to the north and west of these areas mentioned.

 

MN Oak Wilt Spread 2024 (Fig. 5) Illustration by MN DNR

Ready to preserve your oaks?

Whether your oak trees have contracted oak wilt, or maybe you just have some oak trees that you really value, (that is a good thing!), you should reach out to a professional. Not only are we trained and deal with oak wilt on a regular basis, there isn’t anything wrong with a second opinion. Remember that during the high-risk period (April-July) no trimming or pruning should be done, proper measures should be in place if it is not possible to avoid opening wounds during this high-risk period. During this time there should be precautions when removing a tree infected with oak wilt, as it still can transmit through root systems even after the tree has been felled.

If your oaks are in need of some attention, give us a call today. There is no benefit to waiting on your oaks to show more signs, if you suspect there is oak wilt, act swiftly.

Contact us today!

Sources & More Information

Oak wilt. (2024). Minnesota Department of Natural Resources. Retrieved April 4, 2024, from https://www.dnr.state.mn.us/treecare/forest_health/oakwilt/index.html 

Oak wilt in Minnesota. (n.d.). UMN Extension. https://extension.umn.edu/plant-diseases/oak-wilt-minnesota#preventing-below-ground-spread-2352161

Oak Wilt | Forestry and Natural Resources. (n.d.). https://forestry.ca.uky.edu/oak_wilt#:~:text=The%20Oak%20Wilt%20fungus%20(Ceratocystis,detected%20in%20Wisconsin%20in%201944

Oak Wilt – Bretziella fagacearum – Plant & Pest Diagnostics. (n.d.). Plant & Pest Diagnostics. https://www.canr.msu.edu/resources/oak-wilt-bretziella-fagacearum

Oak Wilt Thermal Model – AgWeather. (n.d.). https://agweather.cals.wisc.edu/thermal-models/oak-wilt

Oak Wilt Harvesting Guidelines. (n.d.). Wisconsin Department of Natural Resources. https://widnr.widen.net/view/pdf/aqszuho7ee/Oak-Harvesting-Guidelines-Web-version—FR-560.pdf

Oak Wilt Management. (n.d.). Minnesota Department of Natural Resources. https://www.dnr.state.mn.us/treecare/forest_health/oakwilt/management.html