Plant Problem Diagnostic
Black Knot of Plum and Cherry: Identification and Pruning-First Control
The short answer
Black knot is caused by the fungus Apiosporina morbosa and shows as elongated swellings on twigs and branches that begin soft and olive-green, then harden into the signature rough, warty, coal-black knots that can grow a foot or more long. When a knot encircles a branch it girdles it, so leaves beyond the gall wilt and die.
The fungus spreads from older knots. Iowa State and Wisconsin Extension describe infective spores being released from year-old and older knots during rainy spring weather, then carried by wind and splash to infect young, succulent new growth; the disease develops over two seasons before the black knot is obvious.
The core control is pruning, not spraying. Remove all knots while the tree is dormant in winter, cutting well below the swelling because the fungus extends past it (Iowa State says 6 to 8 inches below, Wisconsin at least 2 to 3 inches), and destroy the prunings by burning, burying, or bagging. Remove wild plum and cherry within a few hundred feet, since they seed reinfection; fungicides are secondary at best and never work alone.
Identifying black knot
The mature symptom is unmistakable, but it starts subtly. First-year infections appear as soft, olive-green to light brown swellings on new twigs, easy to miss. Over the following season these harden and darken into rough, warty, coal-black knots that can reach a foot or more in length and completely encircle a twig or branch. Where a knot girdles the wood, everything beyond it wilts, and repeated girdling over years can hollow out the productive framework of a plum or cherry.
Black knot affects plums and cherries (and other Prunus, including wild and ornamental species). Because the young green swellings are the stage before spores are produced, spotting and cutting them early is ideal, but most gardeners first notice the disease once the hard black galls stand out on bare winter branches, which is actually the perfect time to remove them.
How it spreads
The knots are not just damage, they are the disease's factory. Iowa State explains that infective ascospores are produced on the surface of year-old and older knots and are forcibly discharged during rainy spring weather, then carried by wind and rain splash to infect young, succulent twigs and wounds. The twigs must stay wet long enough for infection to take hold, so wet springs drive bad years.
That two-season, spore-from-old-knots cycle is why sanitation is so powerful. Every knot you leave on the tree, or on a wild plum in the fence row, becomes next spring's source of infection. Cut the knots out before spring spore release and you break the cycle at its source; leave them and no amount of spraying will keep up.
Pruning it out (and why fungicides are secondary)
Prune during dormancy, before spring spore discharge, and cut generously below each knot because the fungus grows beyond the visible swelling. Iowa State specifies cutting 6 to 8 inches below the symptomatic tissue; Wisconsin advises at least 2 to 3 inches below for the same reason. Destroy the prunings by burning, deep burying, or removing them from the property, since knots can still release spores after they are cut off. Where a knot sits on a main scaffold or trunk that you cannot remove, some growers cut the gall out down to healthy wood, though heavily infected trunks may not be salvageable.
Then reduce the outside pressure: remove wild plum and cherry near the planting (Iowa State says within about 500 feet), because these unmanaged hosts constantly reseed the disease. Fungicides are at most a supporting player. Iowa State states they are not recommended because they are not effective on their own, and Wisconsin notes that fungicides such as lime sulfur or copper will not control black knot unless they accompany strict sanitation, applied from before bloom through early shoot growth per label. If you are unsure whether a swelling is black knot or something else like crown gall or an insect gall, your Extension diagnostic lab can confirm before you cut. Fit dormant pruning for black knot into the wider backyard fruit tree spray schedule.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Iowa State University Extension - Black Knot — Black knot forms rough, hardened black knots up to a foot long that can girdle branches; infective ascospores are forcibly discharged during rainy spring weather and infect new growth; prune out knots during dormancy 6 to 8 inches below the symptomatic tissue, destroy prunings, and eliminate wild plum and cherry within 500 feet; fungicides are not recommended because they are not effective on their own.
- University of Wisconsin-Madison Extension - Black Knot, Apiosporina morbosa — Black knot, caused by Apiosporina morbosa, produces galls that are soft and olive-green when new and become hard and black with age; spores from older galls infect young twigs during wet spring conditions; prune during dormancy at least 2 to 3 inches below the swelling because the fungus extends beyond it, and destroy the galls; fungicide alone will not control the disease and must accompany sanitation.