Top: first stages of black knot infection
Bottom: developed black knot infection

BLACK KNOT - A fungal pathogen, Apiosporina morbosa, is a disease that attacks more than 20 species of Prunus including: cherries, plums, flowering almond, apricot, peach, and chokecherry. Rough, black swellings or knots develop on branches over a two-year period.  In spring, the first signs of the fungus can be seen as the bark on the infected twig ruptures and an olive green fungal mass appears.  The knot can grow to be 12 inches in length and two to four times the width of the twig.  As the growth matures it turns hard, black and knotty in appearance.  The twig or entire branches can be girdled (strangled) as the fungus progresses.  Trees may become stunted, ugly and cease fruit production.The spores of this fungus are spread by the wind so as soon as the fungus is detected it should be removed.  Branches attacked by the fungus should be pruned off the plant at least four inches below the black knot.  After each pruning cut is made, pruners should be disinfected with a 10% bleach solution to prevent spreading the fungus.