Growing up in a house heated mainly with firewood meant many weekend trips to the bush to cut trees down and cut the logs into firewood-lengths (not my job), split the largest (sometimes my job), and throw the wood on the truck or tractor bucket. Ever-present in these forest excursions were encounters with shrubs of Prunus, typically chokecherries (Prunus virginiana), infected with black knot. I remember making a point of walking carefully around these shrubs, as the black misshapen growths wrapping the branches were not something I was interesting in having touch me. On my most recent trip to Manitoba a couple weeks ago, I took the time to photograph this phenomenon in a way that I most remembered it — obvious clumps of black something-ness standing out among thin leafless branches, in flat grey light.
The black growths are symptomatic of Dibotryon morbosum infection, as opposed to being (solely) the body of the fungus. Much like cancer in animals (out of control growth of cells), the black knots are the byproduct of too-rapid cellular growth, though in this case stimulated by fungal infection (see: gall). Dispersal of fungal spores from the infected black knots to new green shoots occur in springtime, particularly during periods of warm, wet weather. Small galls can be formed by the end of the first summer, but it isn’t until the succeeding summer that the mass becomes larger and eventually blackens (see: Dibotryon morbosum life cycle).
Black knot (syn. Apiosporina morbosa) was only scientifically described in 1821, well after the initial botanical exploration of eastern North America. From Cornell’s Black Knot Fact Sheet: “The disease can be found throughout North America but is most commonly found in the northeast. It was first reported as a destructive disease in Massachusetts in 1811. It was first described in 1821 by L. D. Schweinitz from specimens collected in Pennsylvania. Researchers believe the disease is caused by a native pathogen that was only found in the northeastern states until around 1875, when observations of the disease began arising in the central states.” In the late 19th century, it was considered “the most destructive disease of tart cherries and plums“, though it no longer seems to have a major economic impact except in ornamental landscapes.