Black Knot

Black knot is a common, often serious, disease of plums and cherries in Kentucky. Trees in both commercial and residential plantings are susceptible to the disease. Black knot results in knotty growths that, over time, encircle limbs and result in branch death.

Black Knot Facts

  • Symptoms begin as small, light brown, irregular swelling or knots on limbs (Figure 1). The next year, enlarging knots become olive-green with a velvety surface and progress to become hardened, brittle, black swellings (Figure 2). Knots, which can reach lengths of 6 inches, continue to expand each year until girdled branches eventually die.
  • Only actively growing twigs of the current season’s growth are susceptible.
  • While infection takes place in spring, knot development is not evident until autumn.
  • The pathogen overwinters in knots on previously infected twigs and branches, and spores are spread by wind and rain.
  • Caused by the fungus Apiosporina morbosa (syn. Dibotryon morbosum).

Figure 1:   Symptoms of black knot begin as small, light brown, irregular swellings or knots on limbs. (Photo: John Strang, UK)

Figure 2: The next year knots enlarge and become hardened, brittle, and blacken. (Photo: Dennis Morgeson, UK)

Management Options

Cultural practices, such as pruning and sanitation, are the primary means for reducing or eliminating black knot.

  • Prune out knots in autumn or winter after leaves fall and infected branches are easy to recognize. Inspect trees again in April and remove any newly formed knots.
  • Remove any trees with girdled trunks and/or large limbs.
  • Destroy or discard all diseased wood.
  • Remove wild plum and cherry trees from the vicinity.

Fungicide applications may be used in conjunction with the cultural practices listed previously, but are often not warranted.

  • Fungicides can be applied in spring to protect young, expanding twigs.
  • Homeowners should refer to Extension publication, Disease and Insect Control for Home Grown Fruit in Kentucky (ID-21) for specific fungicide recommendations.
  • Commercials growers should refer to the Midwest Fruit Pest Management Guide (ID-232) for current fungicide recommendations.
  • Always follow all label directions when utilizing fungicides.

Additional Information

  • Black Knot (PPFS-FR-T-04)
  • Disease and Insect Control Program for Homegrown Fruit in Kentucky, including Organic Alternatives (ID-21)
  • Backyard Peach & Stone Fruit Management Using Cultural Practices (with Low Spray, No Spray, & Organic Options (PPFS-FR-T-22)
  • Homeowner’s Guide to Fungicides (PPFS-GEN-07)
  • Midwest Fruit Pest Management Guide (ID-232)

 

By Kimberly Leonberger, Extension Associate, and Nicole Ward Gauthier, Extension Plant Pathologist

 

Posted in Fruit
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