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Black Knot Fungus

Black Knot Fungus image

Is your Mayday, Chokecherry or Lilac showing a black, lumpy growth on its branches? It may be infected with black knot fungus - a disease that affects Prunus family trees. Although the fungal disease is rarely fatal, if left unchecked it will affect the health of your tree.


This fungal condition infects only Prunus species of plants, and may be recognized by the clumpy-looking, black masses of abnormal growths on the branches of your cherry trees.

What you can do

To manage black knot fungus, it’s essential to prune off infected branches 2-4 inches below each "knot” and dispose of them in a land fill. Between each cut, sanitize your tools with a bleach-water solution (25% bleach, 75% water).

The best time to prune is during late winter, as the fungus is dormant and the abnormal “knotty” growths are easy to see. Avoid pruning in the spring when the fungus is active. The fungus is transported by spores so the proper sanitization of pruning tools is very important to limit its spread from plant to plant.

For the health of the tree, we recommend citizens educate themselves in proper pruning techniques or hire the services of a certified arborist. Pruning a tree or shrub leaves a wound, requiring the tree to heal itself, and correct pruning provides the tree with optimal conditions for healing properly.

For more information about black knot fungus, please visit the Government of Alberta's website.