
Urban species #057: Black knot
Dibotryon morbosumWhat appears to be asphalt spattered on the twigs of a small tree is actually a black knot gall, the reproductive structure of a pathogenic fungus. Black knot only infects trees in the genus
Prunus, a host specificity seen earlier in this journal with
red tree brain.
Prunus trees include several kinds of ornamental cherry trees, as well as plum and peach trees. Black knot is a cause for concern in the American southeast, where it can cause damage to the peach harvest. In the Boston area, black knot is seen on wild cherry tree species such as black cherry and chokecherry.
( one more picture )