Knots frequently become quite numerous and may be found on most of the branches. The first indication of the presence of black knot is usually the swelling of small twigs. As the disease progresses, elongated swollen areas appear on older limbs. These knots are greenish in color and relatively soft in texture when they first appear. As the growing season progresses, the knots become coal black and very hard.
Black knot is caused by a fungus.
The disease gets its start in early spring from spores which are produced on the knots. Spores are carried by wind to other branches where they germinate and cause infection. By early fall, the knots have become visible and during the next season they enlarge rapidly. When they occur on twigs and small branches, prune out the infected branches about two or three inches below the knot. The knots on large branches and trunks can be cut out.