Hi Dave –
Nice question, We just wrote an entire page on how to identify Black Knot and what causes it. This common fungal disease of plum, cherry, apricot and chokecherry trees is easily identified with its hard, uneven, black galls that seem to enwrap twigs and branches. The trick to controlling the fungus is identifying the infection well-before the disease becomes firmly established. Here’s our 7 steps to prevent and get rid of Black Knot on trees.
1.) Inspect your trees carefully for first signs of the disease. This is best done in winter, when leaves are absent, but can be done throughout the growing season.
2.) Remove any knots that are found. Cut well-past the galls, four to eight inches, to ensure all the infection and its spores are removed.
3.) Dispose of infected stems and branches.
4.) Take care not to spread the spores. Clean pruning tools as you use them with a solution of 1/2 cup bleach to a gallon of water.
5.) Copper sprays offered on this website offer significant protection against black knot, but are unlikely to be effective if pruning and sanitation are ignored.
6.) Neem oil can help inhibit the fungus. However, it will not kill fungal spores that are already present.
7.) Remove wild cherry and plum trees from around your property. They harbor the disease and release spores that are easily carried to your susceptible nursery trees.
Hope it helps!