Prevention & Treatment: There are no fungicides specifically available to control stem canker. Keep plants healthy by controlling black spot, powdery mildew and insects. The following cultural methods can help minimize disease development.
Avoid Injury to the Plant During Transplanting, Cultivating, Pruning, & Flower - Cutting: Wounds are a major way the fungus enters the plant.
Prune Properly: To prune an outward facing bud. This will help to avoid too many branches growing into the center of the plant that may cross and rub together.
Remove & Destroy all Infected or Dead Portions of Canes Immediately: Make all pruning cuts well below the diseased areas, and prune about one-fourth inch above an outward-facing bud node, without cutting the nodal tissue, at a 45-degree angle. Prune live canes in the spring, not fall. Disinfect cutting tools after use on a diseased plant in a solution of 1 part household bleach to 9 parts water.
That being said. If the canker problem you are having is a light one. I would recommend using a fungicide that has sulfur in it to help gain control. Such as this one: https://www.planetnatural.com/product/sulfur-plant-fungicide/
