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It is a disease called Black Knot. Black knot of Prunus is caused by the native fungal pathogen Apiosporina morbosa.
Black knot occurs on more than 25 species in genus Prunus (cherry, plum, chokecherry, peach, nectarine, apricot, almond, etc.), but is particularly destructive on ornamental cherry, plum and chokecherry in the landscape.
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