Black knot

Black knot is caused by a fungus called Apiosporina morbosa. The spores of fungus are wind-bourne and can infect anything in the cherry (Prunus) family including plum, Shubert chokecherry, mayday as well as native cherries such as chokecherry, pin cherry and Nanking cherry.

Note that dwarf sour cherries bred at the University of Saskatchewan are resistant to this disease.

Symptoms:

• Black, hard masses that resemble dog droppings encircle twigs and branches.
• Branches will die past the point of the infection once the black knot spreads enough to encircle the branch, called girdling. Left untreated the fungus will spread, eventually killing the tree.

Control:

• Black knot can be pruned out when infections are localized. Late winter is a good time to prune as it is easier to see the knots.
• Prune off the branches at least 10 cm (4") below the black knots.
• To locate a good place to make the cut, find the lower edge of the infection on the branch and then trace that infected branch back to where it is attached to another branch or the trunk. Cut out the branch without cutting into the branch collar. Do not leave stubs.
• Sterilize your pruners in between each cut to avoid spreading the disease. Use a disinfectant like Lysol (at label directions for concentration) or an alcohol-based disinfectant in a spray bottle. Spritz it on your pruning tools and wipe with a soft cloth. Do not use bleach as it will corrode your tools.
• Avoid pruning when the plant is wet as this can also spread the fungus.
• Diseased branches should be bagged and put in the garbage or burned if appropriate.
• If the infection is really severe, remove the entire shrub or tree entirely and replace with a new, unrelated tree such as mountain ash, crabapple, or hawthorn.
• Fungicides are of little value on the prairies as black knot spores are everywhere especially in native populations of prunus species.