University of Maryland Extension

Black Knot- Trees

black galls on twigs
Black knot galls on a twig

 black knot

 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.

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.
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