Black Knot (Apiosporina morbosa)

Black Knot (Apiosporina morbosa)

The photomicrograph below illustrates a stained thin section of plum tree stem infected with Black Knot, a destructive disease of plum trees caused by the fungus Apiosporina morbosa (also known as Dibotryon morbosum). This fungus was first observed in 1821 in Pennsylvania, but can now be found throughout North America. The pathogen also affects apricot, peach, cherry and ornamental trees.