As of April 1 until August 31 it is illegal to prune elm trees within Saskatchewan. This is due to the elm bark beetles being most active during this time. The beetles are attracted to fresh cuts on the tree which helps spread the disease. 

You are allowed to prune before and after this ban period, it helps keep the tree healthy and better able to resist all types of disease including Dutch elm. 

If you suspect an elm to have Dutch elm, you are advised to contact the local municipality. 

More information can be found here