Apiosporina morbosa

Apiosporina morbosa is a fungus that causes black knot, a serious disease affecting wild and cultivated Prunus species in North America. This includes plums, cherries, and apricots.1

The fungus initially infects twigs and branches, leading to swellings that develop into hard, black knots or galls. These knots can girdle and kill the affected parts of the tree, potentially stunting or killing the entire plant if the infection is severe.1

Black knot can be managed by pruning infected branches during winter, removing wild susceptible trees nearby, and applying fungicides to buds. Japanese plum varieties tend to be less susceptible than American and European types.1