Black knot is caused by the fungus Apiosporina morbosa. It derives its name from the black knot-like galls that form on the branches of infected plants.

Plants Affected

  • Prunus species (cherries, plums, apricots, and cherry-plums) are susceptible to this disease; however, in Grande Prairie it is most commonly seen in Schubert chokecherry and Mayday trees.
  • Other potential hosts include pin cherry, chokecherry, sand cherry, Nanking cherry, apricot, plum and flowering almond. However, these plants are rarely, if ever, found with black knot.
  • Amur cherry is considered resistant to the disease.

Symptoms

  • Look for swelling branches that often split the bark in the first year of infection.
  • Second year infection will begin to produce characteristic black knot-like galls on branches.
First year black knot infection (left) causes the branch to swell and deform.  Subsequent years of infection cause the black growths more commonly seen (right).

How it Spreads

  • This disease spreads by wind-blown spores.
  • There is an “obligate moisture period” required for the spores to germinate and establish themselves on new wood, so the disease favours rainy (rather than dry) spring weather.

Controls

Black knot has no registered chemical controls. If you discover black knot in your tree, take the following steps to control it:

  1. Prune infected branches back to the next branch junction at least 30 centimetres (12 inches) below the infected area, preferably back to the next branch.  Late winter after the snow is gone is the best time to do this.
  2. Sterilize pruning tools between cuts as a precaution using gas-line antifreeze or a 10% bleach solution. Soak your pruning tool for at least one minute to ensure you aren't spreading the disease further throughout the tree.
  3. Burn infected material immediately or bag and dispose of it in the landfill. Do not leave galled branches in a woodpile or in your yard, as the galls will continue to release viable spores that can re-infect your trees.

 

Young Tree with Heavy Black Knot Infection