Black Knot of Plum and Cherry
By Dr. Sharon M. Douglas
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8601 Fax: (203) 974-8502
SYMPTOMATOLOGY AND DISEASE CYCLE:
Black knot, caused by the fungus Apiosporina morbosa, is quite common on many flowering Prunus species in the landscape, as well as on wild plums and cherries in woodlots. It is also a very destructive disease of plum and prune trees in orchards. The disease affects only woody tissues and can develop on twigs, branches, and scaffold limbs. Losses result from extensive dieback of girdled limbs and stunting of growth beyond the knots. Trees can be severely weakened, disfigured and, in extreme cases, even killed as a result of infection.
Symptoms are easily recognized as hard, black, elongate swellings or galls on twigs, branches, and trunks of trees. While knots are most outstanding on dormant trees, newly formed knots are greenish and soft but become hard and black with age. The fungus overwinters on infected twigs and produces spores in spring. New shoots are susceptible and can be infected soon after budbreak and throughout the period of active shoot elongation. However, most infections are thought to occur just before bloom or after petal-fall. Wet spring weather is favorable for disease since rain is important for discharging the spores from the knots. In addition, wind and rain help to spread these spores to the susceptible tissues. Spores of black knot are capable of penetrating non-wounded tissues, so they do not require wounds in order to infect.
Most infections occur in spring, but symptoms are often not visible until fall when they appear as small, often inconspicuous swellings on the twigs. These knots gradually enlarge, mature, and take on their diagnostic rough, black appearance during the winter and the following spring. Small twigs usually die within a year of infection, whereas larger branches may live for several years before becoming girdled and killed by the fungus.
Control is best achieved using a combination of culture, pruning and sanitation, properly timed fungicide sprays, and resistant varieties. Pruning and sanitation are essential to any control program since fungicide sprays are relatively ineffective unless old knots are pruned and removed from the vicinity of the tree. Infected tissues should be pruned before budbreak and cuts made at least 6-8" below any visible swellings or knots.
Cultural methods involve removing and/or pruning of any wild plum or cherry trees found in woodlots within 600 feet of the landscape tree in question. These wild trees are highly susceptible to black knot and can be important sources of inoculum.
Resistance is an option for control since edible plum varieties differ in their susceptibility to black knot. The cultivars Stanley, Damson, Bluefree, and Shropshire are considered highly susceptible; Fellenburg, Methley, Milton, Bradshaw, and Early Italian are moderately susceptible; Formosa, Shiro, and Santa Rose are slightly susceptible; and President is considered highly resistant. In general, Japanese varieties are less susceptible than most American varieties.
The final strategy for disease management involves the proper selection, timing, and application of fungicide sprays. Thorough coverage of all parts of the tree is necessary and the sprays should be applied until runoff. The fungicides chlorothalonil and mancozeb plus copper hydroxide are registered for use on many ornamental Prunus species. Applications are usually made when the tree is dormant (just before budbreak) and continued until flowers have fallen (petal-fall). The fungicide label should be consulted for information on specific tree hosts, dosage rates, and safety precautions. If plum, cherry, or prune trees are being grown for edible fruit, please consult the fact sheet Disease Control for Home Plum Orchards. This guide contains information on fungicides registered for use on edible fruit.
Black knot, caused by the fungus Apiosporina morbosa, is common on many flowering Prunus species in the landscape, on wild plums and cherries in woodlots, and is also a very destructive disease of plum and prune trees in orchards. Losses result from dieback of girdled limbs and stunting of growth beyond the knots. Trees can be severely weakened, disfigured and, in extreme cases, even killed as a result of infection. This fact sheet reviews symptoms, spread, and disease management options.