Oak wilt in Minnesota

Oak wilt is caused by the fungus Bretziella fagacearum and is responsible for killing large numbers of oaks every year in Minnesota.

Oak wilt is most severe in the red oak species group such as northern red oak and northern pin oak, and black oak. It also affects white oak species such as white oak, swamp white oak and bur oak, but it kills them more slowly.

Fortunately, several management techniques exist to prevent or control oak wilt.

Oak wilt identification

Leaf discoloration and wilt, tree mortality, and fungal spore mats can indicate the presence of infection. However, several other tree diseases can exhibit similar symptoms. 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.

Oak wilt symptoms in red oak species

• Northern red oak
• Northern pin oak
• Black oak

Tree mortality

• Oak wilt progresses quickly in red oak species and trees do not survive.
• 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.
• Diseased red oaks can often be easily spotted in the summer because of their bright red-brown leaves.

Leaf symptoms

• 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.

Bark and branch symptoms

• When the bark is peeled back from a branch with wilted leaves you may see bluish staining on the wood surface.
• Cracking bark or fungal mats may be present on the tree trunk.

Oak wilt symptoms in white oak species

• White oak
• Bur oak
• Swamp white oak
• Chinkapin oak

Tree mortality

• The disease progresses very slowly in white oak species in Minnesota.
• The disease may develop from year to year with tree death occurring between 2 and 5 years or longer after the first symptoms develop.

Leaf symptoms

• 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.

Bark and branch 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 (seen as rings or narrow discolored wood) may be observed in the cross-section of an infected branch.

Oak wilt spread

Insect transmission is how new oak wilt centers start, but most new infections are caused by the oak wilt fungus spreading through the roots of nearby trees that have grafted together.

Where is oak wilt found in Minnesota?

In Minnesota, the disease currently extends through central Minnesota and encroaches 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.

Spread by insects

• Two species of sap beetles, Colopterus truncatus and Carpophilus sayi, are the primary insect transmitters of the oak wilt fungus from diseased trees to healthy trees in Minnesota.
• Sap beetles are attracted to the chemicals produced by the oak wilt fungal mats between the bark and the wood (cambium) of oak wilt-infected trees.
  • 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 oak wilt mats are commonly produced in spring and fall, the mats produced April through July on red oaks that wilted the previous year are most important in disease spread. During this same period, 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.

Spread through roots

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