Re: When to prune Norway Spruce in lower Michigan
To help prevent a Cytospora infection you should only prune during dry weather. Disinfect your pruning shears between cuts (if you think the tree may already be infected) and always between individual plants (in case one of the spruce is infected). Spruce are less susceptible to infection if they are kept free from stress, so make sure they get enough water during droughts. Here is the Ohio State Fact sheet on Cytospora infection of spruce.