Yes, it’s true! Just as a surgeon will take caution and sterilize her surgical tools so an infection won’t spread from one part of the body to another when doing surgery, so should the gardener sterilize her pruning shears with denatured alcohol so not to spread an infection from one branch to another.
A surgeon wouldn’t use a dull scalpel on her patient, would she? Neither should you use a dull blade on trees or shrubs. Ragged or jagged cuts make it easier for critters and disease to enter into the stem. Cut with clean sharp blades. Remember that rubbing branches can also cause wounds and invite insects and disease. Cut the weaker of the two branches that rub each other. Removing water sprouts on limbs and suckers at the base of plants will also improve their health.
Pruning dead and diseased branches can be done anytime of the year. Just make sure you remove the diseased branch as soon as possible to stop the disease from spreading. It is recommended to cut off 6-12 inches beyond the diseased portion to make sure all the disease is removed. Disinfect shears after every cut and before you put your tool away!
There you go; you have now earned the title of Garden Surgeon!
Check out the Virginia Cooperative Extension Peninsula Pruning Calendar. It is a wonderful resource telling you why, how and when to prune.