Mayday, Chokecherry and other trees in the Prunus family of trees may become infected with Apiosporina morbosa, more commonly known as black knot fungus. 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.
Although the fungal disease is rarely fatal, if left unchecked it will reduce the vigour and ornamental value of your trees. However, infected cherry trees may live for many years and contribute to your landscape. The appearance and vigour of infected trees may be dramatically improved through the removal of diseased branches through proper pruning.
To manage black knot fungus, it’s essential to prune off infected branches 2-4” below each "knot” and dispose of them in a land fill. The fungus spores overwinter in the masses and reproduce in the spring. Pruning should be conducted when plants are dormant. The best time is during late winter when 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 think about hiring 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.