Gardeners beware of Black Knot in trees

black knot

Black knot is caused by the fungus Apiosporina Morbosa and is very common in various species of Prunus, which are ornamental and fruit bearing shrubs and trees such as Mayday, Amur Cherry, and Schubert Chokecherry. The Town of High River prunes trees in public areas for Black Knot twice a year. Residents are advised to check trees in their yards and prune them if necessary. In certain cases, if the disease is allowed to spread, the removal of the tree may be required.


  • Early stages look like small olive-green swelling (galls) on the new growth of a tree.
  • Galls will grow until mature (after two–three years) and will become black, tar-like growths.
  • Mature galls produce and release a vast amount of spores during the bloom period, resulting in a rapid increase in infections. These spores are spread by wind and rain.
  • If the knot circles the entire circumference of the branch, the branch will die. If a tree is severely infected the disease can eventually kill a tree.
  • One diseased tree can contaminate an entire neighbourhood.

What to do:

  • Prune any infected branches during winter or very early spring when plants are dormant and the disease is easy to see.
  • Prune branches 15-20 cm below any visible galls.
  • As a precaution ensure pruning tools are disinfected between cuts.
  • Diseased wood must be destroyed immediately (Burned, buried or removed from site). Diseased knots can produce and release spores for up to four months after removal.
  • Ensure plants are healthy and free from stress (not a guarantee from disease).
  • If caught early enough, Black Knot can be controlled by regular monitoring, pruning and destroying affected branches.

If you have questions or concerns regarding the health of your trees, consult with an International Society of Arboriculture Certified Arborist or equivalent

The Town is currently pruning Elms around town until the end of March. In order to prevent Dutch Elm Disease, pruning of Elms is only permitted from October 1 to March 31. Once the Elm pruning ban begins in April the town will resume pruning for Black knot on town property.

For more information, click here or email Town Arborist or call 403-603-3503 

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