Bur Oak Blight In Minnesota
Bur oak blight (BOB), Tubakia iowensis, is a fungal leaf disease found on bur oak trees throughout Minnesota, including the Brainerd Lakes and St. Cloud areas. It causes leaf browning and leaf loss of bur oak trees in late summer and early fall. Bur oaks can lose about half of their canopy each year and still remain in good health. However, when this happens for several years in a row, a tree may become stressed and more susceptible to two-lined chestnut borer, an insect that feeds underneath the bark of stressed oak trees causing dieback and tree death from the top down.
How To Identify Diseased Oak Trees
Bur oak blight becomes obviously visible in late summer when wedge-shaped areas of the leaf begin to turn brown and small, black spots appear along the underside of the leaf veins. The disease starts in the lower part of the tree and moves upward. Dead leaves attached to the tree throughout winter is another obvious indicator of this disease.
The severity of the disease varies from season to season based on spring moisture. Above normal spring rainfalls for the past 30 years have likely increased the occurrence of this disease in Minnesota. Although infections occur in the spring, trees will appear healthy until late summer when the symptoms progress throughout August and early September.
Effective Injection Treatment for Bur Oak Blight
A fungicide injection after leaf expansion in the spring is the most effective strategy for managing the disease. According to research at Iowa State University, fungicide injections can reduce disease symptoms for at least three years. An insecticide treatment to protect against two-lined chestnut borer should also be applied.