Black knot is caused by the fungus Apiosporina morbosa and attacks various trees and shrubs. It infects trees within the Prunus genus, which includes different types of plum and cherry trees. The fungus has also been known to infect shrubs in the Rose family. The spores spread in warm and wet weather, so it should be removed when temperatures are consistently below zero.
It is spread through spores by rain, wind, birds, and insects that have had contact with infected trees. The spores spread in the spring and infect other trees, then in the summer the infection creates green swellings at the tip of the branches. In the following years, these swellings blacken and enlarge. Learn more and how to control it at https://www.ealt.ca/blog/black-knot-what-is-it-and-why-do-we-care?rq=black%20knot
North Saskatchewan River Valley Conservation Society https://www.edmontonrivervalley.org/