Black root rot, also called dead man’s fingers or Xylaria root rot, is occasionally observed on mature apple and cherry trees. Although trees of all ages can be infected, most trees that die from black root rot are at least 10 years old. Black root rot is caused by two species of the fungus Xylaria, X. mail and X. polymorpha, with the former being more common in the southern Appalachian states, and the latter more common in the eastern states. The disease can occur also on many hardwood species, including black locust, elm, honey locust, maple, oak, hickory, sassafras, and walnut. The fungus produces an off-white internal decay of tree roots and results in thinning of the foliage and tree decline over a period of a few years. As is typical of other root and crown rot diseases, the weakened trees often lean or break near the soil line. See the article on Black Root Rot for more information.
Answer provided by Alan R. Biggs, West Virginia University.