Black Knot of Plum and Cherry
Pest Management Fact Sheet #5091
Authors: Dr. Alicyn Smart, Dr. Bruce Watt, and Abigayl Novak
Other Name: Dibotryon morbosum
Pathogen: Apiosporina morbosa
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 on about 25 species of Prunus.
In the spring, the fungus produces infective spores (ascospores) which are forcibly ejected during rainy periods. The rain splashes these spores and is 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 (). Infection occurs from April through June on the current season’s growth.
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, ascospores are produced again, 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.
- America (Prunus americana), European ( domestica), and Japanese plums (P. salicina)
- Sweet (P. avium) and Mahaleb (P. mahaleb) cherries, and wild and cultivated species of chokecherries
Symptoms and Signs
This disease appears as obvious hard black elongated swellings (knots) which may be 1-6 inches or more in length. These knots are scattered throughout the tree with the number increasing in successive years if the disease 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 corky but become hardened and black by the end of the second year. Diseased twigs often bend due to one-sided overgrowth (Figure 1). Infected branches often wilt, fail to leaf out or die beyond symptomatic tissue.
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 1st. Where infections occur on larger branches, excise infected tissue down to healthy wood.
- Sterilize cutting shears with 70% rubbing alcohol to limit the spread of the disease.
- Burn, bury, or otherwise remove prunings from the area because they may still be an active source of inoculum.
- Severely infected trees should be removed entirely.
- Cut and remove wild hosts of the disease.
- Moderately resistant varieties of plums include Damson, Bluefree, Shiro, Santa Rosa, Formosa, Methley, Milton, Early Italian, Brodshaw, and Fellenberg.
- 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 be generally applied at bud break 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.||Supra Captan 80 WDG|
|Chlorothalonil||See label for spray timing.||Bravo, Echo, Daconil|
|Copper products||Do not apply after full bloom.||Kocide|
|Lime sulfur||On peaches pre-blossom only.||Lime sulfur|
|Sulfur||First spray at pink stage.||Many names|
|Thiophanate-methyl||Apply at early bloom. See label.||Topsin-M|
You should always check your local town ordinance for any pesticide restrictions before application.
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SLJ. Black Knot: Apiosporina morbosa. January 2018. <http://plantclinic.cornell.edu/factsheets/blackknot.pdf>.
Voyle, Gretchen. Our plum trees versus black knot. 7 January 2013. <https://www.canr.msu.edu/news/your_plum_trees_versus_black_knot>.
WHEN USING PESTICIDES, ALWAYS FOLLOW LABEL DIRECTIONS!
Alicyn Smart, DPM
Plant Pathologist and Director of the Plant Disease Diagnostic Laboratory
University of Maine Cooperative Extension
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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