Black Knot

Black Knot, caused by the fungus Apiosporina morbosa, is a very common disease of plants in the genus Prunus. A survey in Alberta revealed a significant and widespread distribution of Black Knot found in commercial, municipal, private and natural plantings. This disease reduces the aesthetic value of affected specimens, as infections spread rapidly; high levels may result in the eventual death of the plant. The most distinguishing symptom of Black Knot is the characteristic black, tar-like swellings that develop on branches of the infected plant.

More information: How to treat Black Knot