Black Knot of Cherry and Plum

by Sherrie Smith and Jason Pavel

Black Knot of Prunus spp. – Apiosporina morbosa

Photo by Keri Weatherford, University of Arkansas
Cooperative Extension Service

Are you noticing black, knobby galls on the branches of your cherry or plum tree?

The common name of the disease is Black Knot, caused by Apiosporina morbosa (Dibotryon morbosum). This is a common fungal disease of Prunus spp.

Yield losses result from extensive dieback of girdled limbs and stunting of growth beyond the knots. Trees can be severely weakened, disfigured, and, in extreme cases, killed because of infection. Prunes, plums, sweet cherries, and sour cherries are all hosts for Black Knot. Rarely do we receive a peach sample with Black Knot. Wild cherries and plums serve as continuous sources of inoculum.

The first symptoms are small, light brown swellings usually located at the base of the leaf petiole or on the fruit spur. These appear during the summer and the first year after infection. Young knots may have an olive-green color, but later become hard, brittle, and black in color. Older knots are coal-black in color and hard in texture. The knots often protrude more on one side of the affected branch.

What are the control measures for Black Knot fungal disease?

Control starts with pruning. Prune out and destroy all visible knots before new growth starts in the spring. The cuts should be made at least 6-8 inches below the lowest part of the knot. Cut out knots on large main branches and trunks with a knife or chisel, including an inch of healthy bark around the knot. Never purchase plants showing knots or abnormal swellings on the twigs and branches. All clippings should be burned, buried, or otherwise removed from the property.

Mancozeb, Captan, Topsin M, or fungicides containing chlorothalonil help control Black Knot if the cultural controls are also practiced. Apply the first spray in the spring just as green tissue begins to appear. Spray again just before and after bloom. Spray at 2- week intervals until new growth stops. Lime-sulfur sprayed during the dormant season is also helpful. When possible remove wild cherries and plums within 600 feet of the orchard to prevent spores from blowing into the orchard and causing new infections. Some Plum cultivars are resistant to Black knot.

Take Aways

• Lime sulfur during dormancy.
• Prune out all galls and dispose.
• Apply the first spray in the spring.
• Spray at 2- week intervals until new growth stops.
• Resistant cultivars are available.

This work is supported by the Crop Protection and Pest Management Program [grant no. 2017-70006- 27279/project accession no. 1013890] from the USDA National Institute of Food and Agriculture.