We love our oak trees and homeowners often ask me what they can do to prevent their trees from becoming infected with oak wilt. One of the most effective preventative measures is to stop pruning by early April. Oak wilt is a fatal disease of oak trees, causing great losses of one of the most beautiful trees in Minnesota. This disease enters and becomes established in a new area when sap beetles, often called picnic beetles, carry spores of the oak wilt fungus on their bodies and feed on freshly wounded trees. The beetles pick up the oak wilt spores when they feed on the sporulating mats produced by oak wilt killed trees during the months of April, May and June. The beetles are attracted to freshly wounded oak trees in their search for food, and thus introduce the spores of the fungus into healthy trees where it establishes. After the fungus enters the tree, the tree attempts to defend itself by producing defense compounds in the water-conducting tissue. These compounds plug the water-conducting tissue causing the tree to exhibit drought-like symptoms and wilt. In red oaks this wilting is very rapid and an infected tree may wilt completely in two to six weeks after the initial symptoms appear. Infected white or bur oak trees die more slowly, a branch at a time, and may survive for several years. Oak trees should not be pruned when the fungus and the beetles are active. Traditionally, the time to avoid pruning has been from early April through early July. New guidelines published by the University of Minnesota in a newly revised publication entitled “Oak Wilt in Minnesota” now divide the calendar into three periods. The first of these periods is the high-risk period which includes the months of April, May and June. Do not prune oak trees during this time. If pruning is unavoidable or if a tree is wounded, paint the wound immediately with a water-based paint or shellac. Apply two coats of the paint or shellac within minutes after the wound has been made. The purpose is to provide a barrier to the fungus, not to help the tree heal. The second of these periods is the low risk period, which includes the months of July, August, September and October. Infection can occur, depending on predisposing weather conditions and insect populations, but infection is rare. Painting the wounds is optional. The safe period for pruning oak trees, as well as American elm trees, is November through March. At this time, the fungal pathogen and the insect vectors are inactive, making it an ideal time for pruning. Painting of the wounds with any kind of paint or wound coating is not recommended. For more information on oak wilt, contact the Stearns County Extension Service and ask for a free copy of the publication, “Oak Wilt in Minnesota.”
