Black knot

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Black Knot is a plant disease caused by the Dibotryon morbosum fungus. It affects the cherry, plum and apricot trees of North America. The disease produces rough, black areas that encircle and kill the infested parts, and provide habitat for insects.

The disease was first described in 1821 in Pennsylvania but has spread across North America. While it was one of the most destructive diseases of the plum and cherry trees in the late 1800s, today it is relatively well controlled and only seen in poorly managed orchards or where strongly established.

Black Knot occurs only on the wood parts of trees, primarily on twigs and branches but can spread to larger limbs and even the trunk. Olive-green swellings from the disease are visible in the late spring, but as it spreads and matures typically by autumn rough black knots circle and kill affected parts. The knots vary in size from one inch to one foot (2.5 cm - 30 cm). Older knots can kill trees by promoting insect infestations.

The most common treatments are pruning infected parts during the winter and spraying buds with a fungicide. Nearby wild plants with the disease are also destroyed.