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Black knot

Quick facts

  • Black knot is a common disease of wild and landscape Prunus species in Minnesota.
  • Spores can be blown from infected plants far away and result in new infections.
  • Knobby swollen black growths called galls grow along the length of stems and branches.
  • Galls can result in leaf wilt, leaf, shoot and branch death, and even death of the tree.
  • Infected branches may distort and bend from the gall's one-sided growth.
  • The cracks formed by a black knot trunk infection can provide an entry point for other wood rotting fungi.
  • Black knot galls can be removed from infected trees through pruning.
Large black knot gall on the main trunk of a tree.
Large black knot gall

Black knot is a fungal disease that mainly affects ornamental and edible plum and cherry trees (Prunus) in Minnesota. Outbreaks are common in both home landscapes and natural areas.

Damage varies greatly between infected trees. Some large shade trees tolerate many galls throughout the canopy with few negative effects.

Galls can cause leaf wilt, leaf, shoot and branch death, and even death of the tree in young or highly susceptible species of Prunus.

  • Make sure ornamental and edible trees and shrubs are disease free before buying them.
  • Choose trees that have some tolerance to black knot when possible.
  • Avoid planting landscape and edible Prunus trees and shrubs in areas where there are many wild trees infected with black knot.
  • If you find only a few infected wild trees, prune out existing galls or completely remove infected plants before planting plum and cherry trees or any other member of the Prunus family.

Identifying black knot


Managing black knot


Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2018

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