I have observed some large shrubs or small trees this winter with masses of what looks like hard black poop on the branches. That growth is black knot of Prunus. It is a fungus, and as the name implies, grows on plants in the genus Prunus. I see this fungal growth mostly on chokecherry. That is no surprise because it is our most abundant and widely distributed Prunus.
Chokecherry is a tall shrub or small tree which grows to a height of 20 feet or so. It can be found across the state in a variety of habitats ranging from prairie hillsides and ravines, to riverbanks and wooded areas. It suckers profusely, and as such often forms small thickets. It provides excellent food and cover for many species of songbirds, upland game birds, small mammals, as well as big game. And of course excellent jams, jellies, and syrups can be made from the fruit, which became our Official State Fruit a few years ago.
But back to the black knot of Prunus. North Dakota is home to several native members of the genus (i.e. wild plum, pin cherry, sand cherry, and of course chokecherry). Introduced species such as Nanking cherry and dwarf Russian almond may also be found in the state, particularly in shelterbelts. All these species are susceptible to black knot of Prunus.
The black elongated swellings or “black knots” on the branches and twigs is often about an inch in diameter, may grow to be several inches long, and are composed of a hard mass of fungal filaments, or hyphae and spores which can be released carried by the wind to other plants. And because chokecherry and other Prunus species are so susceptible and widespread in the region, black knot is also widespread.
Despite being attacked by black knot and several other diseases and insect pests, chokecherry continues to be used in shelterbelts and as a popular ornamental. It is well adapted to our climatic conditions and tolerates a wide range of soil conditions. And its value to wildlife and fruit production are also important factors in its continued use.
But black knot is often a major problem, particularly in lawns and similar situations. It is not only unsightly, it can reduce fruit production, and in cases of heavy infections, trees may become stunted and deformed. It is perhaps the most significant factor limiting the economics of commercial chokecherry production in the region. Infested branches should be trimmed promptly and properly. So you might want to check with your local horticulturalist or NDSU extension agent if you have concerns about black knot.
-Chuck Lura
