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University of Minnesota Extension

Black knot

Quick facts

  • Black knot is a common fungal disease of Prunus trees including ornamental, edible, and native plum and cherry trees.

  • Hard swollen black galls (tumor like growths) form on branches and occasionally on trunks.

  • Many Prunus trees tolerate black knot. Tolerant trees have many galls throughout the tree with few negative effects on the health of the tree.

  • Some Prunus trees are more severely affected by black knot. In these trees, leaves and shoots wilt and die on branches with galls.

  • Management will vary depending on how severely the tree is affected by black knot.

How to identify black knot

Tree trunk with large dark-colored bumpy mass

Branch symptoms

  • Black knot galls are most noticeable during fall and winter after all the leaves have fallen.

  • Knobby, swollen black growths called galls grow along the length of stems and branches.

  • In early summer, young galls or new areas of growth on the edges of older galls are covered with velvety, olive-green spores.

  • These galls turn black and hard by the end of the summer.

  • Infected branches may bend to one-sided due to growth of the gall.

  • There can be anywhere from a few galls to hundreds of galls within the tree canopy.

Leaf symptoms

  • Leaves remain healthy and green even on branches with galls in black knot tolerant trees.

  • Leaves wilt, turn brown and die on branches with galls in trees that are highly susceptible to black knot.

  • Brown, wilted leaves at the end of branches are often scattered throughout the tree on highly susceptible trees.

One young olive green gall and several older black galls on tree branches
A young green gall and older black galls

Trunk symptoms

  • Large areas of rough black swollen bark form on the main tree trunk.

  • Black knot galls on trunks are often cracked and may ooze sticky liquid.

  • Wood decay fungi may enter the trunk through cracks caused by black knot galls and cause wood rot.


How does black knot survive and spread?

  • Black knot is caused by the fungus Apiosporina morbosa.
  • The black knot fungus overwinters in the galls on branches and trunks.

  • Spores are released during wet periods in the spring. The wind carries these spores to trees where they infect young green shoots or wounded branches.

  • The fungus grows within the branch for several months with no outward symptoms of disease.

  • As the fungus grows, it releases chemicals that make the tree grow extra plant cells that are unusually large. This unusual growth results in the swollen, woody galls.

  • Galls are made up of both plant and fungal tissue.

  • One year after infection, galls can be seen as a swollen area of the branch with a velvety olive green covering of fungal growth.

  • Two years after infection, the gall has turned black and hard. These galls release spores in spring when wet.  

  • Sometimes, the branch and the gall die after spores are released in early spring. If the branch lives, the knot keeps getting bigger and produces new spores every spring.

  • The gall can completely encircle and girdle a branch. When this happens, the leaves beyond the gall wilt and die.

  • Although the black knot fungus will not cause the trunk to rot, the cracks from the infection can let in other wood rotting fungi.

How to manage black knot


Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2018

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