Black knot of plum and cherry: what the extension services record
What The Morton Arboretum, the University of Wisconsin-Madison and the University of Connecticut say about black knot: the fungus, the Prunus hosts, how the galls develop over two years, the wild cherries named as a source and the temperatures for spore infection.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page describes what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. None of the three pages read lists black knot as regulated or notifiable. Each section names its source, all read on October 9, 2026. The full list of diseases is on the tree diseases page.
Cause and hosts
The Morton Arboretum and the University of Wisconsin-Madison name the fungus as Apiosporina morbosa, and the Arboretum gives the older name Dibotryon morbosum, the one the University of Connecticut uses. The hosts are trees and shrubs of the genus Prunus. The Arboretum says plum and cherry are most susceptible, peach, nectarine and apricot less commonly infected, and that sweet and sour cherries suffer less than wild black cherry. Wisconsin names wild, fruit-bearing and ornamental plums and cherries. Connecticut calls it a serious disease of plums in that state and says many American, Japanese and European plums are susceptible. The Arboretum says it occurs throughout the United States.
Two sources name wild trees as a source of infection. The Arboretum says it is frequently seen in the woods on wild black cherry and in orchards that are not sprayed regularly. Connecticut says a major source of infection is the native wild cherry along roadsides, as well as wild plums.
How the knots develop
| Stage | What is recorded | Source |
|---|---|---|
| First year | Greenish-brown to brown swelling on a branch or trunk; the bark may begin to split. Usually starts in the newest growth and small twigs die. | University of Wisconsin-Madison; The Morton Arboretum |
| Second year | The swelling enlarges into the hard black gall, commonly called a knot. Connecticut gives about an inch across at first, growing each year. | The Morton Arboretum; University of Connecticut |
| Spring of the second year | The Arboretum says rapidly growing knots are covered with a velvety olive-green fungal growth, larger and darker by summer, hard, rough and black by fall. | The Morton Arboretum |
| Older than two years | Gall tissue often dies and is colonized by other fungi that make it whitish or pinkish. Connecticut says boring insects also invade the knot. | University of Wisconsin-Madison; University of Connecticut |
The Arboretum says infected trees may produce few flowers or fruit, that fruit spurs and sometimes trunks can also be infected, and that the knots eventually cut off water and nutrients to the branch, causing stunting, wilting and dieback. Wisconsin says severe infections can cause general decline or death if galls girdle large limbs or trunks. Connecticut says the knots girdle the stem and the upper part dies, and the Arboretum says it can take several years for twigs or branches to die and that the disease worsens each year unless managed.
Disease cycle and weather
The Arboretum says the fungus overwinters as spores in the knots, released from mature knots from early spring to early summer and carried by wind and rain. The fungus enters the youngest growth through wounds or by penetrating the bark. Connecticut says spores are discharged during spring rain, young growth under three years old is very susceptible, older branches can be infected especially where bark is damaged, and the major infection period begins when buds swell. It adds that a knot takes several months to start developing after infection and usually two years to mature, and that old knots keep producing spores until the branch is dead.
The temperature band differs slightly by source. The Arboretum says most infections occur in wet conditions between 55 and 77 degrees Fahrenheit, and Wisconsin gives wet weather and mild temperatures of 55 to 75 degrees in late spring or early summer. Connecticut's page prints its own temperature figures in a garbled form, so none is quoted here.
Confirmation and status
The Wisconsin clinic page says very few diseases can accurately be diagnosed by eye and describes a paid sample service. The directory lists the plant disease and insect diagnostic labs in the United States. None of the three pages names a quarantine, a notifiable status or a report line.
What the sources say about control
The three sources give different views on fungicides: the Arboretum says the few available are often not effective, Wisconsin says they are expensive and not likely to be effective, and Connecticut says a protectant spray may be needed on the more susceptible plum varieties but will not work without sanitation. This page gives no treatment steps, product names or timings. For another disease of the rose family, see fire blight.