Oak wilt is a fungal disease that causes wilting and eventually death of oak trees. Trees are initially infected by sap feeding beetles like the picnic beetle. Wounds created by storms or pruning serve as a feeding site for these beetles and as an entry point for the fungus.
Once the tree has been infected, the fungus moves through grafted roots to healthy trees nearby. As the tree dies, the fungus reproduces. The fruity smell of fungal mats attracts picnic beetles, which carry fungal spores to fresh wounds.
To prevent infection, avoid wounding trees during April, May, and June (or March when it is unusually warm outside) when beetles are active.
If trees are wounded during these months prevent beetles from interacting with fresh wounds by painting them. Apply a water-based (latex) paint or shellac within minutes. These materials act as a protective barrier; they do not help the tree recover.
Since the chance for infection still exists during July, August, September, and October, the optimum time for pruning trees is when they are dormant.
Manage Existing Infections
To manage existing infections, use a vibratory plow with a five-foot blade to sever below ground root grafts and remove infected trees. Infected trees should be destroyed or covered with heavy plastic through June the following year.
The systemic fungicide Alamo has been used as a preventative treatment in highly valued red and white oaks, and as a curative treatment in white oaks.
For help with identifying, preventing and managing oak wilt infections contact the city forester at 763-717-2660.