Apiosporina morbosa (Schwein.:Fr)
Distribution: Widespread; the disease is very damaging on many varieties of plum, less so on sour cherry.
Black knot usually develops over two seasons. The disease first appears in late summer or autumn as an olive-green swelling on new shoots (A). Disease develops rapidly the following summer, forming a characteristic dark, course-textured warty knot (B, C). Knots vary from 2.5 cm to nearly 30 cm in length and may or may not encircle the branch. The vascular tissue becomes restricted in infected branches, ultimately leading to the death of the branch.