Plant Disease Clinic Highlights

Black Knot

Black knot occurs on cultivated and wild cherries and plum. The disease is caused by the fungus Apiosporina morbosa. Black knot is characterized by the presence of rough, warty black galls which may extend up the branch from a few inches to a foot or more. Fungal spores produced on year-old or older galls initiate infections on young, succulent twigs.

Control:

Sanitation measures usually control the disease adequately, but may be supplemented with a fungicide program if the disease is unusually difficult to control.

  1. Prune out and destroy all galls while trees are dormant. Cuts should be made 6-8 inches below the visible infection.
  2. Use resistant cultivars in new plantings.
  3. Fungicides such as lime sulfer or tribasic copper sulfate will aid in control of black knot. Several applications need to be made beginning before bloom, and continuing until after fruit set. Spraying alone will not provide satisfactory control.

This article originally appeared in the April 27, 1994 issue, p. 56.

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