Black Knot of Plum and Cherry

Fruit Disease Information Note 4 (FDIN 004)
D.F. Ritchie, Plant Pathologist


The disease black knot is caused by the fungus Apiosporina morbosa (sym. Dibotryon morbosum). Black knot can occur on both sweet and sour cherry, Damson, American, European, and Japanese varieties of cultivated plums and prunes as well as wild cherries and plums. The disease is common throughout much of North Carolina occurring on many wild plum and cherry trees. These serve as an annual source of infection for cultivated varieties. Black knot occurs on the woody parts of the tree such as twigs, limbs and sometimes the trunk. The knot is unsightly and often becomes infested by borer-type insects. The disease may become very destructive causing death of effected twigs and limbs, and occasionally entire trees. There is great variation in level of susceptibility in wild cherry.

Symptoms

The common name "black knot" describes the main symptoms of the disease. However, the first symptoms are small, light brown swellings usually located at the base of the leaf petiole or on the fruit spur. These appear usually during the summer and 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 (hence the name "black knot") and hard in texture. The knots often protrude more on one side of the affected branch.


Figure 1. Black knots on affected trees. On left, knot is on the main trunk, on right, swellings are on individual branches.

Life Cycle

Black knot infections are initiated by spores which germinate and penetrate the current year's growth. The spores come from either galls formed on the cultivated plum or cherry or from galls on wild plums and cherries in the vicinity. Infection can occur from the green cluster stage of bud development until terminal growth ceases in early summer (April-June). At the green cluster growth stage, the leaf buds show 1/4 to 1/2 inch new growth and the bloom buds are exposed, but tightly grouped. After infection, several months are required before a light brown swelling can be seen. Later the same year or the next spring, the swelling turns olive-green and produces spores within one to two years after the initial infection occurred. These spores are spread by wind and rain to twigs on the same tree and nearby trees.       

Control

A. Destroy Sources of Infection

   1. During fall and winter, or before new growth starts in the
      spring, prune out and destroy all visible knots.  On large, main
      branches and trunks, knots should be cut out with a knife or
      chisel. One inch of healthy bark around the knot should also
      be removed.

B. Obtain Disease-Free Stock

    1. Never purchase nursery stock showing visible knots or
       abnormal swellings on the twigs and branches.

    2. When pruning branches with knots, the cut should be made
       at least 2-4 inches below the lowest part of the knot.

    3. Because spores can develop and spread from knots left on
       the ground or in brush piles for several weeks after removal
       from the tree, they should be burned or buried.

C. Chemical.  Fungicide sprays applied in a timely, regular manner can prevent most new infections. Use Benomyl (Benlate 50% wettable powder) at the rate of 1/2 tablespoon/gallon of water. Spray schedule:

    1. Apply first spray in the spring just as green tissue begins to
       appear.

    2. Again just before and after bloom.

    3. Spray at 2-week intervals until terminal growth stops, usually
       early to mid-June.

D. Resistance.  There is some variation in resistance between varieties, but it does not hold up in all areas.

Other Links

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For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service

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Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies. Last printed 04/91

Published by North Carolina Cooperative Extension Service. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914; Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Reformatted Dec. 2000 by A.V. Lemay