Oak wilt in Minnesota

In Minnesota, the disease is currently found in an area bounded on the north by Pine County, on the west by Stearns and Nicollet counties, and south to the Iowa border.

The greatest concentrations of oak wilt are found in Sherburne, Anoka, Isanti and northwestern Dakota counties.

The known range in Minnesota where oak wilt is threatening shown in red areas. Map updated June 2021.

Red oak group

• Following infection, the fungus is quickly transported through the water-conducting system of red oaks and leads to rapid wilting.
• Wilting usually starts at the top or outer portions of the tree crown and quickly progresses downward.
• Leaves take on a bronze to reddish-brown discoloration beginning with the tip and margins, progressing toward the midrib and base of the leaf.
• A water-soaked appearance may develop on dark green leaves. Affected leaves drop off quickly and can be found on the ground around the dying tree.
• Complete wilting and leaf loss can occur in as little as 4 weeks in branch infections or shortly after leaf-out the spring following root graft infection.
• A dark bluish-gray discoloration may be observed on the wood surface when bark is peeled back from a branch with wilted leaves.

White oak group

• Affected branches of bur oaks are scattered through the crown.
• Progressive development of the disease may occur year to year with tree death occurring between 2 and 5 years or longer after first symptoms develop.
• Bronzing and browning of leaves generally occurs from the tip and a portion of the leaf margin toward the midrib or base of the leaf, but symptoms may be irregular.
• In white oaks, a single main branch or fork of the crown may exhibit wilting leaves during summer but may develop no further symptoms until the next year or following years.

White oaks in Minnesota have been observed with very slowly progressing symptoms. 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 may also be observed in the cross section of an infected branch.

Oak wilt lookalikes

• Bur oak blight, another common and significant disease of bur oak in Minnesota, can be readily confused with oak wilt.
• Injury caused by two-lined chestnut borer can also be confused with oak wilt.
• Anthracnose may mimic some leaf symptoms of oak wilt, but usually occurs only in the lower crowns of trees.

Spread by insects

Two species of sap beetles are the primary insect transmitters of the oak wilt fungus (C. fagacearum) from diseased trees to healthy trees in Minnesota.

Insect transmission is how new oak wilt centers are started.

Sap beetles are attracted to the chemicals produced by the sporulating fungal mats in the bark-wood interface (cambium) of oak wilt-killed trees.

Although mats are commonly produced during the spring and fall, the mats that are produced April through mid-July on red oaks that wilted the previous year are most important in disease spread.

This is the same time period during which red oaks produce large diameter springwood vessels that are particularly susceptible to infection by the oak wilt fungus.

In addition, sap beetles are strongly attracted to tree chemical compounds associated with fresh, wood-penetrating wounds. Thus, wounded oak trees visited by fungus-contaminated beetles can result in oak wilt spread, particularly during the spring months.

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/wound sealer or shellac.

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

Timely removal and proper treatment or disposal of diseased oaks is also critical for preventing insect spread. This is most important for oak wilt-killed red oak species because fungus mats are commonly produced on them.

Spread by root grafting

The oak wilt fungus spreads from diseased to healthy trees either below-ground via connected roots or above-ground by insects.

Most new infections are caused by the oak wilt fungus spreading through 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 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, including species from different oak groups.

Stop below-ground spread

Root grafts are most common between closely-related oak species (e.g., red oaks). Healthy trees of a different species can be found in oak wilt infection centers (e.g., bur oaks in a red oak infection center).

Cutting root connections between diseased and healthy oaks is the best way to prevent expansion of existing oak wilt centers.

• A vibratory plow with a 5-foot long blade is commonly used to cut the roots.
• You 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.

Hire an experienced tree care professional to mark lines for root cutting.

• 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.

After establishment of control lines, wilting and recently wilted red oaks should be felled and debarked, burned, buried or wrapped and sealed in 4-6 mil plastic until the end of September the year following tree wilt.

If not destroyed, the spores that may form on these oaks the following spring could be carried by sap beetles to wounded oaks and start new infection centers.

Trees that wilted during the growing season should be felled in the fall or winter and either treated on the property or promptly transported to an approved wood waste utilization site.

Options for treatment on the property include debarking of the trunk, burying the main stem and large branches, or cutting logs into firewood lengths and stacking to allow for drying.

If diseased trees or firewood are not removed before spring, the cut and stacked logs should be covered with 4-6 mil clear plastic and sealed at the ground line by late March of the year following tree wilt to prevent beetles from reaching the spore mats. The plastic then can be removed at the end of September of the year following tree wilt and the logs can be safely used for firewood.

Firewood

• Don't move logs or firewood from recently wilted oaks to areas where oak wilt is not present. Oak wilt mats may form on these logs.
• Long distance movements of firewood has resulted in the establishment of oak wilt in distant areas that previously had been unaffected by the disease.

Systemic injection with propiconazole by qualified arborists may prevent 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 movement of oak wilt through oak roots, and is not a substitute for severing root grafts.
• Propiconazole treatment of white oaks 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.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.