Michelle Grabowski, UMN Extension Educator
Although very few plant diseases are active in the landscape during the cold winter months of Minnesota, winter is a good time to check trees and shrubs for branch infections like cankers and galls.
One common gall forming disease of Prunus species is black knot. This fungal infection causes swollen lumpy wooden black growths to form along the length of infected branches. In the winter, with no leaves to hide them black knot galls stand out and can be easily found within the canopy of infected Prunus trees.
Black knot is caused by the fungus Apiosporina morbosa, and infects over 25 species of Prunus. This includes fruit bearing trees like sour cherry (P. cerasus), European plum (P. domestica) and American plum (P. americana), as well as wild Prunus species like pin cherry (P. pensylvanica) and choke cherry (P. virginiana). Ornamental trees and shrubs like flowering almond (P. triloba) and purple leafed plums (P.cerasifera) can also be infected. There are a few disease resistant fruit tree varieties, but unfortunately most are not hardy in Minnesota.
Black knot galls are elongate, knobby growths that grow parallel to the length of the stem. When young, galls may appear olive to brown. They then turn black when mature, and may crack and crumble with age. Galls are a combination of fungal tissue and wood, so that galls will be hard.
The black knot fungus initially infects young growing shoots or wounded stems in spring. Spores are released from mature black knot galls and splashed by rain or blown by the wind to infect new branches. Infected branches do not begin to swell until late summer or early the next spring, and it will be one more growing season before those galls are mature and capable of releasing spores themselves.
The damage done by black knot can vary greatly depending on the species, variety and intended use of the tree. Some ornamental Prunus trees can have a canopy full of galls and still produce a beautiful flower show in the spring and a healthy flush of foliage in the summer. In other trees, galls can distort twig growth and eventually girdle the twig, killing all leaves and branches beyond the gall. Multiple infections of this type eventually reduce the vigor of the tree and send the tree into decline. Occasionally black knot infections can be found on the main trunk of the tree. These infections are swollen and covered with lumpy black gall like growth. They often crack and ooze sap. Although the black knot fungus will not cause trunk decay itself, the cracks formed by a trunk infection can provide an entry point for other wood rotting fungi.
March is the best time to prune out and destroy black knot galls. Make the pruning cut 4 inches below the visible gall. This will insure that all of the fungal infection has been removed. Galls should not be left near the tree, since spores can be produced even on pruned branches. Instead remove or destroy the infected branches.