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Fact Sheets - Black Knot of Plum and Cherry

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Pest Management Fact Sheet #5091

Black Knot of Plum and Cherry

Bruce A. Watt, Extension Plant Pathologist

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Introduction

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.

Environmental Conditions

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.

Symptoms

This disease appears as obvious hard black elongated swellings (knots) which may be 1-6? 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.

Control

Black knot can be controlled using a combination of prevention and sanitation.

  1. Remove all knots and swellings by pruning 3-4? 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? from its edge.
  2. Burn, bury, or otherwise remove prunings from the area because they may still be an active source of spores.
  3. Severely infected trees should be removed entirely.
  4. Cut and remove wild hosts of the disease.
  5. Use resistant varieties if disease pressure is high.
  6. 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

 References

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 Office
491 College Avenue, Orono, ME 04473-1295
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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Published and distributed in furtherance of Acts of Congress of May 8 and June 30, 1914, by the University of Maine Cooperative Extension, the Land Grant University of the state of Maine and the U.S. Department of Agriculture cooperating. Cooperative Extension and other agencies of the U.S.D.A. provide equal opportunities in programs and employment.

Call 800-287-0274 or TDD 800-287-8957 (in Maine), or 207-581-3188, for information on publications and program offerings from University of Maine Cooperative Extension, or visit extension.umaine.edu.


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