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 more common in southern Appalachian states and the latter more common in 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 (fig. 1) 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.
| Figure 1. Thinning foliage due to Xylaria root rot. Photo: John Hartman, University of Kentucky. |
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The disease gets its name from the black sheath of fungal tissue found on the surfaces of decaying roots. Xylaria produces clusters of black, finger-like or club-shaped fruiting structures that originate on the lower trunk or on major roots (fig. 2). These fungal structures resemble blackened “fingers,” inspiring the common name “dead man’s fingers.” They are unique and easy to recognize when they appear at the base of the tree, in the lawn, or in a nearby landscape bed. Trees usually become infected when their roots come into contact with roots or root pieces colonized by Xylaria.
Managing the disease is difficult because the fungus persists on root fragments for as long as 15 years after the removal of an infected tree. For this reason, where a tree has died or has been removed because of black root rot, the site is generally undesirable for planting or replanting with apples. If replanting is necessary, deep plow the site, try to remove all remaining roots, and leave it fallow for as long as possible. Avoid planting susceptible trees on sites with a history of this disease. Apple rootstocks can vary in their susceptibility, although none are resistant. MM.104 and MM.111 are more susceptible than MM.106 and seedling rootstocks. Peaches are not susceptible and would be a good choice for black root rot sites, so long as the soil is deep and well-drained.
| Figure 2. Fruiting bodies of Xylaria polymorpha resemble blackened fingers. Photo: John Hartman, University of Kentucky. |
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Alan R. Biggs, West Virginia University