Black Knot Fungus
Those ugly black lumps in your maydays and choke cherry trees could be a fungal disease called Black Knot. Winter is the best time to see and prune it. Branches pruned are best destroyed by burning.
The Alberta Government website has great information about how to identify and control this disease. If you suspect a tree on your property has black knot fungus, local companies can be contacted for help with identification and pruning.