black growth on tree branch - black knot disease

Black knot on Prunus sp. Photo: William M. Brown Jr., Bugwood.org

Updated: March 9, 2021

Key points

  • Black knot, caused by the fungus Apiosporina morbosa, forms galls on plants in the genus Prunus, including plum, cherry, flowering almond, apricot, and blackthorn.
  • The first symptoms appear in the fall as swellings on twigs.
  • The swellings enlarge the next season, gradually encircle the stem, and form black galls.
  • The surface typically splits or cracks, giving a roughened or crusty appearance.
  • The stem beyond the gall typically dies back and fails to leaf out.
black knot on cherry

Black knot on cherry tree branch. Photo: David L. Clement, University of Maryland

Management

  • Chemical controls are not effective.
  • Prune out infected stems and branches at least 4 inches below the galls and remove the clippings from the landscape before April 1.
  • Infected wild plum and black cherry are often a source of new infections.