Before your Prunus sp. shrubs and trees are fully leafed out, you can easily see if there are any Black Knot infections. Monitor gardens and adjacent wild areas for large black growths on previous years twigs. Prune the cankers out, back quite close to the main stem BEFORE LEAVES EMERGE. Destroy all pruned twigs. Pruning too shallow retains the undetectable, developing canker on the tree and does nothing to limit the disease.
For more information from the Ontario Ministry of Food, Agriculture and Rural Affairs, click here.