Black Knot of Prunus

I remember as a kid exploring the woods around my home and seeing what looked like black poop on some twigs of an occasional large shrub or small tree. I remember looking up at that stuff and wondering what kind of an animal would climb up on those branches to go to the bathroom. I was curious, but not curious enough to climb up and get a closer look.

That was my introduction to what is commonly referred to as black knot of Prunus. No doubt many of you were similarly introduced to this fungus when you were a kid. Although readily observed year around, it’s particularly noticeable in the spring as we begin to spend more time outside before the trees and shrubs leaf out.

Black knot of Prunus, as the name implies, is a fungal parasite that is specific to plants in the genus Prunus. The most common host in our region is chokecherry, but we also have some wild plums and a couple other cherry species.

The characteristic elongated swellings or “black knots” on the branches and twigs of the host plant are typically about an inch in diameter and may be two to six inches long. In old severe infestations they may be considerably longer. They consist of a hard mass of fungal filaments, or hyphae and spores. These spores then spread by the wind to other plants.

Chokecherry is native to our region, and has also been widely used as an ornamental. More recently however, problems with black knot have reduced its desirability in lawns and boulevards. You may recall that chokecherry was designated the state fruit in 2007. Black knot can reduce fruit production, and in cases of heavy infections, trees may become stunted and deformed. Black knot is perhaps the most significant factor limiting the economic feasibility of commercial chokecherry production.

The next time you see these black knots, your thoughts may wonder to a childhood memory, or perhaps of chokecherry jam, syrup, or wine. Whatever your thoughts, scouting out black knot this spring may help you find a few new chokecherry plants to harvest this summer.

Chuck Lura

Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Minot State University-Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.