Black Knot of Plum and Cherry
Pest Management Fact Sheet #5091
Bruce A. Watt, Extension Plant Pathologist
Black Knot is one of the most common diseases of plum and cherry (rare on other Prunus spp.) in Maine. It is caused by the fungus Apiosporina morbosa and can severely limit the production of fruit trees or ruin the esthetic value of ornamentals.
In the spring, the fungus produces infective spores (ascospores) which are forcibly ejected during rainy periods. These spores are splashed by the rain and blown by the wind to land on susceptible plant tissue. The spores can germinate and infect new tissues during wet periods as short as 6 hours at the optimal temperature for infection (70-75°F). Infection occurs from April through June on the current season’s growth.
This disease appears as obvious hard black elongated swellings (knots) which may be 1-6 inches or more in length. The knots are scattered throughout the tree with the number increasing in successive years if the condition is left untreated. When the symptoms first appear during the autumn following infection, the knot appears as an inconspicuous swelling on the current season’s shoots. As growth resumes the following spring, the bark splits and knots are greenish and soft but become hardened and black by the end of the second year.
Survival and Dispersal
The fungus over-winters in the stem of the infected host, resuming growth during the spring. During the second year, infections produce asexual spores (conidia) which are considered unimportant in the disease cycle. As the knot darkens through the summer and the following winter, sexual spores (ascospores) are produced and it is these spores which cause most infections. If the knot has girdled the stem sufficiently to cause its death the infection will stop. Otherwise the knot will continue to expand and produce new spores in successive years.
Black knot can be controlled using a combination of prevention and sanitation.
- Remove all knots and swellings by pruning 3-4 inches below the knot during the dormant season before April 1. Where infections occur on larger branches, excise infected tissue down to the wood and 2-3 inches from its edge.
- Burn, bury, or otherwise remove prunings from the area because they may still be an active source of spores.
- Severely infected trees should be removed entirely.
- Cut and remove wild hosts of the disease.
- Use resistant varieties if disease pressure is high.
- Preventative sprays may be necessary if nearby disease sources cannot be eliminated or when bringing a heavily infected tree back to health. A dormant spray of lime sulfur may be helpful when pruning heavily infected trees. Fungicides, which have been effective against black knot, should generally be applied at budbreak and every week to two weeks, especially before rain, until terminal growth stops.
|Fungicide||Application Notes||Examples of Trade Names|
|Captan||See label for timing.||Captan|
|Chlorothalonil||See label for spray timing.||Bravo, Echo, Fung-Onil and others|
|Copper Products||Do not apply after full Bloom.||Bordeau Mixture, Kocide, Tenn-Cop, Liqui-cop, Basicop, and others|
|Lime sulfur||On peaches pre-blossom only.||Lime sulfur|
|Mancozeb+copper hydroxide||For use on ornamentals.||Junction|
|Sulfur||First spray at pink stage.||Many names|
|Thiophanate-methyl||Apply at early bloom. See label.||Topsin-M|
Ogawa, J. M. 1995. Compendium of Stone Fruit Diseases. American Phytopathological Society, St. Paul, MN
McFadden-Smith, W., Northover, J., and Sears, W. 2000. Dynamics of ascospore release by Apiosporina morbosa from sour cherry black knots. Plant Dis. 84:45-48.
Rosenberger, D. A, Meyer, F. W, and Engle, C. A. 1994. Fungicides for controlling black knot on plums. Meeting of the American Phytopathological Society, Northeastern Division. Phytopathology, Volume: 84 Issue: 11 Pages: 1375
Latham, A. J. and Campbell, H. L. 1994. Long-term fungicide control of black knot of plum. Annual Meeting of the American Phytopathological Society. Phytopathology Volume: 84 Issue: 10 Pages: 1078
Rosenberger, D. A.and Gerling, W.D. 1984. Effect of black knot incidence on yield of Stanley prune trees and economic benefits of fungicide protection. ; Plant Disease, 68, 12, pp 1060-1064
Sinclair, W. A. and H. H. Lyon. 2005. Diseases of trees and shrubs. Cornell University Press, Ithaca, NY. 660 pp.
When Using Pesticides
ALWAYS FOLLOW LABEL DIRECTIONS!
Pest Management Unit
Cooperative Extension Diagnostic and Research Laboratory
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1.800.287.0279 (in Maine)
Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.
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