Sometimes the galls will have a whitish powdery coating. Fungal spores are produced on galls that are at least one year old. Young succulent twigs are the most likely candidates for new infections. Newly formed knots are greenish and soft, but blacken with age Good sanitation through gall removal is the primary control measure. Pruning, preferably during dormancy, should be done at least 2-3 inches below the swellings, since the fungus may extend beyond the visible portion of the black knot.
Sanitation usually keeps the disease in check.
Fungicides may aid in disease control, but are not typically necessary. Fungicides used without good sanitation will not control the disease. Diseased wild plums or cherries nearby, such as in wooded areas or fence rows can be a spore source, so keep an eye on them, too. Chances are that one-day you will unexpectedly happen upon this peculiar formation and you will know exactly what it is. Prepared by Barbara Ambruzs , extension program specialist, Department of Plant Pathology Iowa State University , Ames, Iowa.