Black knot is caused by the fungus Apiosporina morbosa. Fungal spores produced on year-old or older galls initiate infections on young, succulent twigs. The knots are soft when newly formed and later become hard and black with age. Often the knots become covered with a whitish secondary fungus or insects.
The disease is controlled primarily through good sanitation measures. All galls should be pruned out and destroyed, preferably when the tree is dormant. It is important to make cuts and least 2-3 inches below the swelling because the fungus may have extended beyond the swelling.
Sanitation measures usually control the disease adequately, but fungicides such as lime sulfur or tribasic copper sulfate may aid in control of black knot. Several applications need to be made beginning before bloom, and continuing until after fruit set. Fungicide use alone will NOT control the disease.
It is also helpful to remove any diseased wild plum or cherries in the vicinity, such as in fence rows or nearby wooded areas.
This article originally appeared in the May 9, 1997 issue, p. 61.
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