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.
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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