Look closerFungi
Black knot on cherry trees: the black lumps are a fungus
When: All year
Those hard, black, rough swellings wrapped right around the twigs of a roadside cherry are black knot, a native fungus called Apiosporina morbosa, not fire damage and not an insect gall. The knot is a living thing that keeps growing along the same branch year after year, and on a black cherry it can go on enlarging for decades.
Why this time of year
The knots are there all twelve months, because they are a swelling of the living branch rather than a mushroom that comes and goes. What changes is whether you can see them: UMass Extension says the dormant season is the best time to scout for black knot, when there is no foliage to hide the galls, so from leaf-fall to leaf-out is when a branch full of them stops being invisible.
How to notice it
- Find a wild cherry along a roadside, an old field edge or a woodland edge, and look at the twigs and smaller branches rather than at the trunk.
- Look for hard, black, rough-surfaced swellings that wrap most or all of the way around a twig, often several inches long and often cracked.
- Follow a knotted branch outward past the swelling. Growth beyond an old knot is often stunted or dead, because the gall can choke off what is past it.
- If you come back in summer, look for a white dusty coating on an established knot. That is a second fungus parasitizing the first one, and it does not attack the tree at all.
Try this with kids: That branch has a hard black lump growing right around it. Do you think something burned it, something bit it, or something is living in it?
a gall is a swelling of a plant's own tissue, grown in response to something else living in it, here a fungus rather than an insect
Sources
- umass.eduUMass Extension, the Massachusetts source and the card's spine. Black knot of Prunus is caused by the native fungal pathogen Apiosporina morbosa. On black cherry, very large galls develop over years to decades on primary branches or the main trunk. Established galls are thickened, abnormally shaped and black. The galls continue to grow and expand as long as the stem tissue remains alive. Later in the summer, established galls may take on a white, dusty coating, which is the result of infection by a mycoparasitic fungus that is not a pathogen of the host. And the scouting note: the dormant season is the best time to scout for black knot when foliage is not present to obscure the galls.
- netreefruit.orgThe New England Tree Fruit Management Guide, published by UMass Amherst, so a second university extension publication rather than a trade guide. The fungus overwinters on infected trees and is common on wild cherry and wild plum. Ascospores are released during rain beginning soon after budbreak, continuing through shoot elongation, with most just before and during petal fall. This page also describes first-year infections as GREEN swellings on new shoots that are easy to miss, which is where it differs from the Morton Arboretum page below.
- mortonarb.orgThe Morton Arboretum, cited only for range-wide pathogen biology and for the synonym: the fungus is Apiosporina morbosa, synonym Dibotryon morbosum, and it is frequently seen in the woods on wild black cherry. Its stage sequence is light-brown swellings by autumn, velvety olive-green growth the FOLLOWING spring, larger and darker over the summer, hard rough and black by fall. Nothing Illinois-specific is taken from this page and UMass independently confirms the same pathogen on the same host in Massachusetts.
Checked against these sources on September 13, 2026.