Black Knot is a fungus that is easily spread by wind and rain. Fungal spores enter the tree through new shoots or wounds. Infections can be seen as early as the fall of the same year or in the spring of the following year. At first, knots are olive green, but will later turn to black.
When knots become visible, prune at least 6" (inches) below the infection. Remember to sterilize your pruner after every cut in a 1 part bleach to 4 parts water mixture. Pruning will not make the tree knot free; however, you will slow the spread of Black Knot throughout the tree.