Bur oak mortality and dieback

This year there was an unusual amount of branch dieback and tree mortality in bur oak across a wide area of southern Minnesota. There were basically three patterns of this dieback and decline.

1. Many of these trees exhibited the symptoms of oak wilt in parts of their crowns but did not test positive for the disease. Much of this branch dieback may be related to environmental stress such as the record cold winters and cool late springs we have experienced over the last few years.
2. There was an unusual amount of oak wilt disease in bur oak suggesting the disease may be latent in this species, with symptoms expressed in certain years.
3. There was some well defined dieback in stressed mature bur oaks probably caused by Hypoxylon atropuctatum. This fungus is known to be a saprophyte on several species of hardwoods and as a pathogen it almost never acts alone. That is, it is known to be associated with the dieback of stressed trees. This fungus invades the bark and sapwood of stressed trees causing a canker, which then causes the dramatic dieback symptoms in the crown above. It can be identified on the bark of dying bur oaks by the large black stromata (flat and hard fruiting body of the fungus) found on dead bark of the canker face.