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Black knot

Infects

Plum, Cherry, Chokecherry, Pincherry and Wild Cherry trees

Symptoms

At the point of infection, a green swelling appears on the newest growth on a branch in the autumn. As the knots or gnarled portions mature, they turn black and are covered with the fungus that causes this disease.

As the disease progresses, the infection spreads and knots can occur on all sizes of twigs and branches. Often the diseased twig bends sharply at the knot.

If the knot circles the entire circumference of the branch, the branch will die.

Severely diseased trees may become worthless because of the amount of dieback.

Cause

Black knot is caused by a fungus that is spread by wind and rain to twigs where the infection takes place. Once the fungus begins growing on a black knot, it produces fruiting bodies that spreads the disease.

Black knot can also be spread by contaminated pruning equipment.

Cure

If caught early enough, black knot can be controlled by pruning and destroying the affected branches.

IPM Toolkit

Preventive tools

  • Sterilize your pruning equipment with methanol (gas line antifreeze) between cuts.

Physical tools

  • Each winter or early spring, prune any infected branches. The branch should be pruned at least 10 cm below any visible swelling since the fungus extends out into the branch further than the knot.
  • Dispose of any diseased branches by taking the infected wood to the landfill.
  • Severely infected trees may need to be removed to prevent spreading to other trees.

Biological tools

  • Not applicable

Chemical tools

  • Spray tree with lime sulfur according to the label directions when pruning. You should spray 3 times during the season:
    • When the trees are dormant (just before buds break)
    • When trees are at full bloom stage
    • When trees are at the late blossom stage (only a few petals remain)