So if you amputate promptly, you can slow down the spread of the disease. In most cases, pruning out the dead limbs will only delay the inevitable. In other words, if a tree wasn't already very stressed out, it wouldn't fall victim to these fungal scavengers. Unfortunately, once a tree becomes severely stressed, it's often difficult or impossible to reverse the slide. Fungicides are ineffective against cankers, and genetically canker-resistant blue spruce varieties are a wish rather than a reality.
Prevention is a large part of the solution to the canker threat.
The way to canker-proof a tree is to protect it from severe stress. The most common culprit in the Midwest is drought stress. Research has shown that drought-stressed blue spruces are vulnerable to Cytospora canker, but trees that receive enough water don't develop cankers even when inoculated with the canker fungus. We can't avoid drought in the Midwest. But we can shield blue spruces from severe drought stress.