Oak wilt in Minnesota

In Minnesota, the disease currently extends through central Minnesota and is encroaching on the northern forest. The greatest concentrations of oak wilt are found in Sherburne, Anoka, Isanti and northwestern Dakota counties.

 Check the DNR website for a map of the current known range of oak wilt disease. The known range in Minnesota where oak wilt is threatening is shown in shaded areas.

Red oak group (northern red, northern pin, and black oak)

Oak wilt progresses quickly in red oak species. If infected in the branch, complete wilting, leaf loss, and tree death can occur in as little as four weeks. If infected by root grafting, complete wilting and tree death can occur shortly after leaf-out the following spring.

• Look for leaves with brown edges and green centers. Leaves will continue browning toward the center and base of the leaf. 
• Wilted leaves drop off quickly and can be found on the ground around the dying tree.
• Wilting usually starts at the top or outer portions of the tree crown and quickly progresses downward.
• When the bark is peeled back from a branch with wilted leaves, you may see bluish staining on the wood surface.

White oak group (white, bur, swamp white, and chinkapin oak)

White oak species in Minnesota have been observed to have very slow disease progress. Progressive development of the disease may occur year to year with tree death occurring between 2 and 5 years or longer after the first symptoms develop.

• Look for leaves with brown edges and green centers. Discoloration generally occurs from the tip and edges toward the middle or base of the leaf, but symptoms may be irregular.
• Wilted leaves drop off quickly and can be found on the ground around the dying tree.
• Symptoms often initially appear in one to several scattered wilting branches, but may progress slowly and develop no further symptoms until the next year or following years.
• A dark brown to black discoloration on the wood surface may be found when the bark is peeled back from a branch with wilting leaves. 
• Walled-off fungal infections (seen as rings or narrow discolored wood) may be observed in the cross-section of an infected branch.

Twolined chestnut borer injury can be confused with oak wilt and probably mimics oak wilt the most. Both problems start at the edge of the crown and kill oaks. Rapid leaf drop and near total leaf loss within a couple months is the best distinguishing characteristic for oak wilt. Branches and tree crowns infested with twolined chestnut borer hold onto their dead leaves for a relatively long time.

Bur oak blight is another common and significant disease of bur oak in Minnesota. It can be readily confused with oak wilt but starts in the lower crowns of trees. Like oak wilt, leaves have dead, brown areas, and many prematurely fall. However, with BOB, the splotches on the leaves have better defined edges, and often are shaped like a wedge. Another sign it is BOB rather than oak wilt is seeing more leaf loss on the inside and lower parts of the canopy rather than the edges.

Anthracnose may mimic some leaf symptoms of oak wilt but usually occurs only in the lower crowns of trees. Infected leaves are often distorted, cupped or curled. Severe infection can result in leaf drop in spring. Trees produce a second growth of leaves by midsummer if leaf drop occurs.

Get an accurate diagnosis of the disease before taking any control action. Hire an experienced tree care professional or consult the University of Minnesota's Plant Disease Clinic.

How is oak wilt spread by insects?

 Insect transmission is how new oak wilt centers start. Two species of sap beetles, Colopterus truncatus and Carpophilus sayi, are the primary insect transmitters of the oak wilt fungus (B. fagacearum) from diseased trees to healthy trees in Minnesota.

• Sap beetles are attracted to the chemicals produced by the oak wilt fungal mats that lie between the bark and the wood (cambium) of oak wilt-infected trees. In addition, sap beetles are strongly attracted to tree chemicals associated with fresh, wood-penetrating wounds. So, wounded oak trees visited by fungus-contaminated beetles can result in oak wilt spread, particularly during the spring months.
• Although mats are commonly produced in spring and fall, the mats that are produced April through July on red oaks that wilted the previous year are most important in disease spread. This is the same time period during which oaks produce large diameter “spring wood” vessels as part of growing wood that are particularly susceptible to infection by the oak wilt fungus.
• Oak bark beetles (Pseudopityophthorus species) are important oak wilt carriers in some parts of the U.S., but not in Minnesota.

Prevent spread by insects

• Avoid wounding or cutting healthy oaks, particularly during spring and early summer, to prevent fungus spread by the sap beetles. 
• If you must prune branches or cut down trees, immediately treat the cut surface with water-based paint, a pruning or wound sealer, or shellac.

Risk of oak wilt fungus spread by sap beetles

In general for Minnesota, the risk level is SAFE from November through March, HIGH from April through July, and LOW from August through October. But in spring risk level can vary considerably based on seasonal change and location.

University of Wisconsin Extension provides a webtool to assess the risk of oak wilt based on the predicted presence of the sap beetles in spring. Always check the model for your date and location before pruning or wounding oaks.

* Exact dates for the beginning and end of each time period may vary from year to year. 

Diseased wood treatment and removal

Timely removal and proper treatment or disposal of diseased oaks are also critical for preventing insect spread. This is most important for red oak species killed by oak wilt because fungus mats are commonly produced on them. Check with a tree waste disposal site or with your local county, city, or municipality for properly disposing diseased wood.

Spread by root grafting

Most new infections are caused by the oak wilt fungus spreading through the roots of nearby trees that have grafted together. 

• Root grafting depends on the oak species involved, the size of the trees, soil type and terrain. For example, root grafting is very common among northern pin oaks on sandy soils in flat terrain. The maximum distance over which root grafting may occur is also dependent on these same factors.
• As a general rule, the more distance between diseased trees, the less likely the disease will spread by root grafting. For example, the majority of such spread in a Minneapolis-St. Paul urban study was found to occur within 30 feet. But researchers found that wilt did occur in some trees up to 50 feet from the nearest infected tree.
• Root grafts may occasionally occur between different species of oak, such as between northern pin oaks and bur oaks.

Stop below-ground spread

Root grafts most commonly occur between closely-related oak species (red oaks). Healthy trees of a different species can be found in oak wilt infection centers (bur oaks in a red oak infection center). Cutting root connections between diseased and healthy oaks is the best way to prevent the expansion of existing oak wilt centers, where oak trees initially died from oak wilt. 

Hire an experienced tree care professional to mark lines and complete the root-cutting procedures. Use the following techniques and procedures:

• A vibratory plow with a 5-foot-long blade is commonly used to cut the roots.
• Your tree care professional can use other equipment such as a trenching machine, backhoe and mini-excavators, but they are more disruptive to the site, require back-filling with soil, and often do not reach a 5-foot depth.
• Careful digging with shovels may be necessary in situations where oaks are near houses, retaining walls or other structures.
• The primary control line is generally placed between the first and second ring (or tier) of healthy oaks out from the diseased trees. This is because the healthy-appearing trees closest to the diseased trees may already have the fungus in their roots, even if they appear non-symptomatic.
• When only using a primary control line, the healthy oaks within that line can be removed after root cutting is finished. Or, they may be monitored for several years and removed if they wilt.
• A secondary control line may be placed between the diseased and healthy trees to preserve additional trees. This secondary control line often fails though and complicates management efforts.

Wood from infected oaks

Trees that wilted during the growing season should be felled in the fall (after root cutting) or winter (if no root cutting takes place) and either treated on the property or promptly transported to an approved wood waste utilization site.

Options for treatment on the property include debarking the trunk, or cutting logs into firewood lengths and stacking to allow for drying.

Avoid moving logs or firewood from infected oaks during the high risk period. If diseased trees or firewood are not removed before the high risk period begins in spring, the cut and stacked logs should be covered with 4-6 mil clear plastic tarp by late March of the year following tree wilt to prevent beetles from reaching the spore mats. Completely bury edges of the tarp to prevent sap beetles from coming into contact with spores. The tarp should be thick enough to prevent punctures.

Outside of the high risk period, you can split oak into firewood segments no wider than 4 inches and pile loosely before January to allow logs to dry out before spring. You can also burn, debark, and chip infected logs and branches, or kiln-dry or process logs into lumber before April. Chips and bark will not spread infection, so they can be left on site. For areas where oak wilt is common, leave infected trees standing for one year after death.

Firewood

DO NOT move logs or firewood from recently wilted oaks or areas with infected oaks. Oak wilt mats may form on these logs. Long-distance movement of firewood may result in the establishment of oak wilt in distant areas that previously had been unaffected by the disease.

Systemic injection with propiconazole by qualified arborists prevents oak wilt symptoms for up to two years in healthy oaks if the oaks are not already infected with oak wilt.

• Propiconazole will not prevent the movement of oak wilt through oak roots and is not a substitute for severing root grafts, so root cutting is still necessary to stop the outward progression of the disease.
• Propiconazole treatment of white oaks (both Q. alba and Q. macrocarpa) already exhibiting early symptoms of oak wilt (less than 30% of crown affected) can prevent further disease development for at least two years.
• Treatment of red oaks already showing symptoms is not recommended, as therapeutic treatments have frequently failed.