The fungus, Dibotryon morbosum , causes this potentially serious disease which affects both wild and cultivated species of chokecherry to domestic varieties. Yield is reduced since severely infected trees are stunted and because control necessitates pruning to remove the knots. Symptoms Rough, elongated, hard, black swellings commonly on twigs but also on branches or stems are characteristic of this disease. Usually knots occur on one side of the twig but occasionally branches become completely girdled killing the portion above the infection.
New infections show up as green swellings which enlarge, develop cracks and turn black with age.
Old black knots may be partially covered by a white to pinkish mold and be riddled with insect holes. Disease Cycle Winter spores formed in mature black knots are spread by wind and rain to twigs where infection takes place through unwounded tissue. Infection continues to occur until terminal growth stops and it is most severe when conditions are mild and wet. Only several months after initiation of infection do green swellings become visible and usually not until spring. These newly formed knots produce summer spores.