What is Black Knot?
Black Knot, caused by the fungus Apiosporina morbosa, is a very common disease of plants in the genusPrunus (See Table 1). 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.
Plant Species Affected by Black Knot
Amur Cherry, Mayday Tree,
Apricot, Mongolian Cherry,
Black Cherry, Nanking Cherry,
Chokecherry, Pin Cherry,
Dropmore Cherry, Cultivated Plum,
Flowering Almond, Wild Plum,
Flowering Plum, Prunus Hybrids,
Japanese Plum, Sand Cherry,
Korean Cherry and Sour Cherry
How can you recognize Black Knot?
The most distinguishing symptom of Black Knot is the characteristic black, tar-like swellings that develop on branches of the infected plant.
Initially, a small, olive-green gall or swelling will develop at a succulent growing point or fruit spur (as a result of spores landing and infection taking place). This swelling will grow until it is mature after 2-3 years. The mature galls are hard, black, 10 to 15 cm (4 to 6 inches) and may be somewhat ruptured. Mature galls will produce and release a vast amount of spores during the bloom period, resulting in a rapid increase in infections. The fungus continues to grow internally and externally, with the branch eventually becoming girdled and dying.