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.
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.
A. Destroy Sources of Infection
1. During fall and winter, or before new growth starts in the
B. Obtain Disease-Free Stock
1. Never purchase nursery stock showing visible knots or
2. When pruning branches with knots, the cut should be made
3. Because spores can develop and spread from knots left on
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
2. Again just before and after bloom.
3. Spray at 2-week intervals until terminal growth stops, usually
D. Resistance. There is some variation in resistance between varieties, but it does not hold up in all areas.
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
Reformatted Dec. 2000 by A.V. Lemay