Black knot disease of cherries and plums
Black knot disease, caused by the fungus Apiosporina morbosa, is widespread on scattered wild and cultivated cherries, chokecherries, and plums in central Minnesota. It causes black, warty stem galls or ?knots? which may vary from one half inch to more than one foot in length. The knots can eventually encircle and cause death of branches or stem.
Spores released from the knots during spring infect green shoots and stem wounds. This parasitic fungus secretes indoleacetic acid and other growth-regulating chemicals that stimulate cambial activity and swelling of the infected area. At maturity (fall of the second year after infection) each knot?s surface is covered with the fungal fruiting bodies that release the spores. After spore release, the black part of the knot dies but growth of the knot at its margins can produce new spores the following spring.
Black knots and other suspicious swellings should be removed and destroyed before budbreak. Cut at least six inches into the healthy wood below the infection. Follow pruning with a dormant application of lime sulfur to the entire crown before budbreak. If buds have opened plan to spray next year. Topsin M or Fungo can also be applied when leaf buds just show a green tip, when blossoms are in early bloom stage and again when petals fall to prevent infection.