Pat, 

Thank you for your question about black knot.  I am not sure what kind of trees you have, so I want you to know the information I found was about black knot on plums and Cherries.  This disease is a fungus and is spread by wind and weather.  It causes stunted growth, and over time can kill the affected tree.  There was no information about pruning, but since the fungus spreads in other ways, not pruning is no guarantee that trees will avoid infection if exposed. Two articles of particular help were found at <extensionpsu.edu>under the search topic 'black knot'.  If this has not been of help or answered your question, please feel free to contact me with further information.