Calgary has had a widespread epidemic of black knot for well over fifteen years and the problem is only getting worse. Apiosporina morbosa is the Latin name for this fungal pathogen which predominately spreads during cool moist weather conditions in the spring. Control of the disease in infected trees is best done by pruning out symptomatic branches that appear as if they have a burned marshmallow on them. Black knot does infect trees systemically which means that even if infected branches are pruned the disease will continue to proliferate within the tree until it is dead and removed.
Because of the natural riparian areas within the city of Calgary it is nearly impossible to completely control the disease as it spreads on native plant species most commonly the western chokecherry Prunus virginiana. The best way to cure tree diseases including black knot in Schubert chokecherry and mayday species is to plant disease resistant trees, in other words the answer is simple do not plant mayday or chokecheery trees because they are guaranteed to get black knot. If you have a tree with a severe infection consider removing and replacing the tree rather than delaying the inevitable. Trees that only have a handful of knots appear per year can be pruned and preserved until the infection worsens.
Keeping a tree healthy will also ensure better chances of success, water regularly when dry, improve soil and site conditions if possible, prune symptomatic and dead branches, avoid over fertilisation and high nitrogen fertilizers.
