Black Knot of Prunus
Nicholas Brazee, UMass Extension Plant Pathologist
Black knot galls on shaded interior
branches of a plum (Prunus domestica) Black knot galls on an ornamental cherry
Cherry stems growing at distorted
angles due to black knot galls The white, dusty coating on black knot galls
is from a mycoparasitic fungus attacking
the black knot pathogen Enormous black knot galls in the canopy
of a forest black cherry (Prunus serotina)
Severe outbreak of black knot on an
ornamental plum (Prunus domestica)
Black knot of Prunus, caused by the fungal pathogen Apiosporina
morbosa, is a common and destructive disease in our region.
The winter months present a good opportunity to scout for and
prune infected stems from the canopy, when they are more read-
ily visible.
As the name indicates, the disease affects species in genus
Prunus. In landscape settings, the most common hosts are plum,
cherry and chokecherry. Infections develop on young and succu-
lent shoots during the spring, but the symptoms are often not
visible until the following year. The initial symptoms appear as
irregular swellings on the stems, but these are easily missed
without careful inspection. After one year, these infected areas
develop into black, gall-like cankers, which are a mixture of
ungal and host tissue.
10 MAA NEWS / March 2020
Cankers can girdle small stems outright or become perennial
on larger branches. In some cases, basketball-sized galls develop
over the course of many years. When advanced infections
develop in the canopy, serious dieback occurs and trees can die.
If detected early, cankered stems can be pruned and destroyed,
but regular scouting is essential. Prune several inches away from
the visible canker to ensure the pathogen is eradicated. With reg-
ular scouting and pruning, the disease can be controlled in many
cases. Prunus should be planted in full sun to avoid the disease,
as deeply shaded settings are conducive to fungal infections.
For more information about black knot of Prunus,
see the following fact sheet at
http://ag.umass.edu/landscape/fact-sheets/black-knot-of-prunus