Black knot is primarily a disease of cherry and plum trees among other members of the Prunus genus. The name “black knot” aptly describes the most obvious sign of this disease: a rough black gall that can encircle the entire branch. Infection by the fungus Apiosporina morbosa occurs during the spring, but the symptoms do not appear until the following spring. At that time, the bark on affected branches ruptures and a green liquid is expelled, and an olive-green swelling is formed.
Over the summer, the gall turns a darker color, eventually becoming black and hard. While the older parts of the gall die subsequently, the fungus continues to grow both around and along the affected branch. Large branches can resist the disease a long time after the infection begins, thereby supplying spores that enable continued spread.
The most effective control method is simply pruning out the infected areas of a tree. In severe infestations, removal of the entire tree is often necessary. There are also certain cherry varieties that are more resistant to black knot to consider when planting.
It is important to sterilize pruning tools used when working around black knot so the disease is not spread to healthy parts of the tree.