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Black Knot Disease

Black knot disease is a problem in our community. There are many options for addressing this problem on your property.

What is Black Knot?

Black Knot, caused by the fungus Apiosporina morbosa, is a common disease of plant species of the following kind:

  • Amur Cherry
  • Apricot
  • Black Cherry
  • Chokecherry
  • Dropmore Cherry
  • Flowering Almond
  • Flowering Plum
  • Japanese Plum
  • Korean Cherry
  • Mayday Tree
  • Mongolian Cherry
  • Nanking Cherry
  • Pin Cherry
  • Cultivated Plum
  • Wild Plum
  • Prunus Hybrids
  • Sand Cherry
  • Sour Cherry

What does Black Knot look like?

The most noticeable symptom of Black Knot is is the black, tar-like swellings that develop on branches of the infected plant.

 

Black Knot Disease

Initially, a small, olive green growth will develop at a succulent growing point or a short twig that bears fruit buds. This swelling will grow until it is mature after 2 to 3 years. The mature growths are hard, black, 10 to 15 cm and may be somewhat ruptured. They will produce and release a vast amount of spores during the bloom period, resulting in a rapid increase of infections. The fungus continues to grow internally and externally, with the branch eventually becoming covered all around and dying.

What can you do to control the disease?

Remove infected branches to at least 6 to 8 inches below the knot. Prune infected branches back to an appropriate location, such as a healthy collar, rather than leave a stub. 

All cutting blades must be cleaned and disinfected after pruning, especially if cuts have been made through obviously infected material.

For knots on scaffold branches or trunks that can't be removed, cut away diseased tissue down to good wood and at least 1/2 inch beyond the edge of the knot.

Failure to remove branches beyond the internal growth will result in re-growth of the fungus.

Black Knot control

All diseased wood should be burned or buried. Do not place wood in the organic bins. Diseased knots can produce and release spores for up to 4 months after removal.