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 side due to the growth of the gall.

• There can be anywhere from a few 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.

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.

Trees affected by black knot in Minnesota

Most susceptible (likely to be damaged by black knot)

• American plum (P. americana)

• Canadian plum, including ‘Princess Kay’ (P. nigra)

• Chokecherry, including: ‘Shubert’ and ‘Canada Red’ (P. virginiana)

• European bird cherry (P. padus)

• European plum, including: ‘Stanley’ (P. domestica)

• Japanese plum (P. salicina)

• Purple-leafed plum (P. cerasifera)

Less susceptible (may be infected but often tolerates black knot)

• Nanking cherry (P. tomentosa)

• Pin cherry (P. pensylvanica)

• Sargent cherry (P. sargentii)

• Sand cherry (P. pumila)

• Sour cherry (P. cerasus)

• Western sand cherry (P. purmila var. besseyi), including: Purple leaf sand cherry Prunus x cistena

Rarely affected

• Amur chokecherry (P. maacki)

• Apricot (P. armeniaca)

• Flowering almond (P. triloba)

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 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.