Black Knot Fungus

How to recognize and control the disease

Type of Problem: Fungal

Range: Widespread, particularly in communities adjacent to natural areas.

Plants Affected: Mayday, Schubert Chokecherry.


Large, highly visible, elongated, rough, knotty swellings appear on twigs and branches. In spring, the knots are a velvety, olive green colour; later in the season, knots become hard and tar black. The swellings arise along the sides of larger branches or may wholly girdle twigs and branches. Knots continue to expand each year, giving a grotesque appearance to branches. If the tree has not died after several years of infection, it likely to be severly stunted and unproductive.


The fungus produces spores in winter and summer; control programs are aimed at winter spores, which infect trees just before they are in full bloom. In late winter, prune infected branches and twigs 4 to 6 inches below the knots.