What is Black Knot?
Black Knot, is a disease commonly found on plums and Prunus species. The fungus causes the plant to produce unusual galls which eventually grow to girdle twigs and branches, restricting water supply and killing portions of the canopy. It is spread to other plants via wind and water.
Control
Manage the disease by carefully pruning away the affected tissue. Use sterile pruning tools to make a cut roughly 4 to 6 inches past the lowest point of symptoms on the branch. After cutting the branch, sterilize the tools again by dipping them in a 10% bleach solution (1 part bleach, 9 parts water -> e.g. 1/2 cup bleach, 4 & 1/2 cups water). You should also spray a bit of this bleach solution on the cut wound on the tree. Repeat this process until all the black growth has been removed. Burn the branches and do not compost them. Prune during dry weather to prevent the spread of the disease. This can be done in the fall and winter if the fruit is still growing on affected branches. The client may need to repeat this process every winter for the rest of the tree’s life or until symptoms no longer appear.