Black Knot – Apiosporina morbosa – Dibotryon morbosum

Black knot Common name Black knot Causal agent Black knot is a fungal disease caused by the fungus belonging to the genus Dibotryon. It is common in all kinds of Prunus trees, whether ornamental or edible. Scientific name Apiosporina morbosa (Dibotryon morbosum) Symptoms & Signs The main feature of black knot disease is the swollen, hard, black tumor-like structures called galls on branches and trunks. It is covered by spores during the early summer and causes the branches to tilt to one side due to their weight. If the tree is not tolerant, the branches with galls die, and the leaves wilt and turn brown. The galls may also secrete a sticky liquid. Transmission The galls are covered by spores which are released during the spring. The wind carries the spores to new, young trees or those with open wounds, where it infects those trees. The infected tree produces extra cells, which are condensed to form galls. With time, the galls change their color from light brown to olive green and finally black. Rainfall highly favors the release of spores which causes this disease. Time of concern Early spring to late summer Common hosts Plum Prune Cherry Peach Apricot