Black Knot Disease otherwise known as Dibotryon morbosum, is a serious, yet fairly common fungal disease of plums and cherry trees (including choke cherries). Other members of the Prunus family (like peaches, nectarines, apricots and almonds) can also be affected but it is less common. The affected trees develop hard, black elongated swellings, called knots, thus the name black knot. These knots are scattered throughout the tree and can girdle branches. If the disease is left unchecked it can kill the entire tree.
The disease is characterized by the presence of thick black, irregular, tumor like swellings on the twigs. They are particularly noticeable in winter.
Symptoms of black knot first appear as light brown, swellings about 1 cm long on the shoots of the current season’s growth in late summer or the following spring. In the second year the knots enlarge. At the beginning of the season they appear like small olives, but by the end of the second year they have enlarged up to 6 inches, hardened and turned their characteristic black colour. The knots can appear on one side of the branch or totally encapsulate the branch and can be scattered throughout the tree. Smaller twigs usually die within a year after being infected. Larger branches may live for several years before being girdled and killed by the fungus.
The pathogen (Apiosporina morbosa), over winters on the black knots. In the spring, when warm rainy conditions prevail, the spores (ascospores) are forcefully discharged and carried by the wind and rain to the young green growth of uninfected branches or nearby susceptible trees. Ascospores that land on them may germinate and cause infection if the twigs remain wet for 6 hours or more.
Management of the Disease
Plant resistant varieties: According to the University of Maine Cooperative Extension Moderately resistant varieties of plums include Damson, Bluefree, Shiro, Santa Rosa, Formosa, Methley, Milton, Early Italian, Brodshaw, and Fellenberg. (Dr. Alicyn Smart, Dr. Bruce Watt, (2019)).
Pruning: Prune out infected areas during dormancy (before ascospore discharge). This includes the small brown swellings, cracked areas and most importantly the black knots. Prune several inches back from the point of infection as the fungus spreads beyond the knot. (Note: Some researchers advise pruning back 6″-8″from the knot while others advise 2″4″. As a general rule, when I am pruning for black knot the bigger the knot the farther I prune back.) Be sure to disinfect your pruning tool between cuts. Rubbing alcohol works well as does Lysol spray or bleach. If major scaffold limbs or the trunk have been infected chisel out the diseased tissue plus an extra 1cm (½”) beyond. Burn, bury or black bag all plant debris. If the disease is to wide spread consider removing the tree entirely.
- Lime sulfur (1 part lime sulfur to 8 parts water) sprayed before bud break, then a second spray at full bloom (1 part lime sulfur to 50 parts of water) and finally a 3rd spray at shuck fall (1 part lime sulfur to 50 parts water) Government of Canada (2014).
- Captan (2 tbsps. per gal. of water) sprayed at full bloom and shuck fall.
- Bordo (a copper based fungicide) applied before bud break serves to provide an inhibitory barrier. Following is a recipe for Bordo mix from the Gov. of Canada. (2 oz. copper sulphate to 1 gal. of water and 3 oz of hydrated lime in 2 gal. of water. Pour the copper sulfate solution into the lime water and strain through a cheese cloth. Use the solution immediately after mixing and also fresh lime is essential not some left over from the previous season).
Note: Shuck fall is when the papery shuck (outer covering) around the exterior of developing fruits drops off the fruit.
Photo credits: all photos by the author.
Dr. Alicyn Smart, Dr. Bruce Watt, and Abigayl Novak (2019); University of Maine Cooperative Extension: Insect Pests, Ticks and Plant Diseases Pest Management Fact Sheet #5091 ‘Black Knot of Plum and Cherry’; Retrieved on Jan. 11, 2021 from: https://extension.umaine.edu/ipm/ipddl/publications/5091e/
Government of Canada (2014); ‘Black Knot of Plum and Cherry’; Retrieved Jan. 11, 2020 from: https://www.agr.gc.ca/eng/agriculture-and-the-environment/agricultural-practices/agroforestry/diseases-and-pests/black-knot-of-plum-and-cherry/?id=1198101468695
Ministry of Agriculture Food and Rural Affairs (2009), Ontario Tender Fruit IPM ‘Black Knot’; Retrieved on Jan. 11,2020 from: http://www.omafra.gov.on.ca/IPM/english/tender/diseases-and-disorders/blackknot.html
Updated on Feb. 28, 2021
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