Black Knot of Plum and Cherry

The fungus, Dibotryon morbosum, causes this potentially serious disease which affects both wild and cultivated species of chokecherry to domestic varieties. Yield is reduced since severely infected trees are stunted and because control necessitates pruning to remove the knots.

Symptoms

Rough, elongated, hard, black swellings commonly on twigs but also on branches or stems are characteristic of this disease. Usually knots occur on one side of the twig but occasionally branches become completely girdled killing the portion above the infection. New infections show up as green swellings which enlarge, develop cracks and turn black with age. Old black knots may be partially covered by a white to pinkish mold and be riddled with insect holes.

Disease Cycle

Winter spores formed in mature black knots are spread by wind and rain to twigs where infection takes place through unwounded tissue. Infection continues to occur until terminal growth stops and it is most severe when conditions are mild and wet. Only several months after initiation of infection do green swellings become visible and usually not until spring. These newly formed knots produce summer spores. The fungus extends several inches beyond the knots and knots will expand with age.

Control

  1. Clean up trees annually. This involves pruning out swollen areas during winter or early spring. Prune at least 3 inches below the visible swelling as the fungus extends several inches beyond the knot. Wounds should be covered with a wound dressing, such as Braco, shellac, Bordeaux paint (Bordeaux powder plus linseed oil mixed into a paste) or other reliable wound dressing products available from seed and garden supply dealers. Destroy all prunings by burning before spring since pruned knots will still produce spores which can spread the disease. Severely infected trees should be removed and burned.
  2. In establishing new plantings and also in helping to control disease in established plantings, wild cherries and plums in the vicinity should be thoroughly cleaned up or removed.
  3. The above control measures will usually be sufficient to keep the disease under control, however, where the disease is a severe and continual problem additional control measures may be needed.

In this regard, thorough applications of several fungicide sprays, along with diseased wood removal will help control the disease. Spraying alone will not control the disease. However, when using chemical control read the product label carefully since it shows the purpose for which the chemical is sold, directions for use, and handling precautions.

Dormant Spray

  1. Spray just before buds break in the spring with one of the following: 2 tablespoons/gallon (=2 lb/100 gal.) of thiram, or lime-sulfur solution (1 part lime-sulfur to 8 parts of water), or 4:6:100 Bordeaux Mixture (see below for preparation).
  2. At least two additional sprays at Full Bloom and at Shuck Fall. Use one of the following: 2  tablespoons/gallon of captan, or 2 tablespoons/gallon of thiram, or lime-sulfur solution (1 part lime-sulfur to 50 parts of water).
  3. A small volume of 4:6:100 Bordeaux Mixture is made by dissolving 2 ounces copper sulfate in one gallon of water and 3 ounces of hydrated lime in two gallons of water. Pour the copper sulfate solution into the lime water and strain through fine cheesecloth. Use the solution immediately after mixing and also fresh lime is essential not some left over from the previous season.
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