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Black Knot Disease

Black knot disease is caused by a fungus and affects a wide range of prunes species including plums, cherries and apricots in Alberta. The disease is extremely aggressive on mayday and Shubert chokecherry trees. The disease has progressed over the last decade and is now causing significant problems with trees.

The Black knot fungus infects trees between April and June during warm, wet weather. The first symptom is a small, light brown swelling on the current or last year’s growth. These swellings are difficult to notice at first until the following spring when they enlarge and appear as an olive green velvety knot. During this stage the fungus produces spores which are released at or near the time the trees are beginning to leaf. Spore release is heaviest at blossom time. Soon after, the knots become darker and by fall, they appear as the black knots that are atypical of the symptoms. The disease may not be recognized until one or two years after the original infection. Knots will continue to grow until they girdle the branch and kill it. Severe infections can eventually kill the tree.

Management of the disease

  1. Examine your trees yearly for the presence of Black knot. Remove all infected branches from any diseased trees adjacent to the property if possible. Prune out and destroy infected branches before bud break. Prune at least 2-4 inches below each knot because the fungus grows beyond the edge of the knot itself. If pruning or removal of major limbs is not possible then you must cut away the knot down to healthy wood at least 1/2 inch beyond the knot. Infected trees usually require pruning on an annual basis until the disease is completely eradicated.
  2. Avoid planting new plums, prunes or cherries next to or downwind from areas with a significant black knot problem.
  3. Fungicides may offer protection against Black knot, but are likely to be ineffective if pruning and sanitation are ignored. These treatments are generally not recommended in urban areas.