Basic Info:

Black Knot is caused by the fungus Apiosporina morbosa which is a very common disease of plants in the genus Prunus. The most distinguishing symptom of Black Knot is the characteristic black, tar-like swellings that develop on branches of the infected plant.

What We Recommend:

1) Remove the infected tree to discourage infection from spreading to other branches/trees.

2) Prune out all knot-bearing branches during late fall, winter or very early spring when plants are dormant and knots are easy to see. Remove infected branches to at least 15-20 cm (6-8 inches) below the knot. First year growth is extremely hard to identify as it presents as just a small swelling and is easily missed even with the best inspections. NOTE: It is preferable to prune an infected branch further back to an appropriate location, such as a healthy collar, rather than leave a stub. This must be done at consistent intervals to keep disease populations to a minimum.

Trees Effected by Black Knot:

Prunus Species

Government Informational Source Link: https://www.alberta.ca/black-knot