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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

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 olive 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.

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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

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Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2018

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