Both edible and ornamental cherry trees are susceptible to this fungal disease. If caught early, black knot is easy to control. This is a fungal disease with symptoms of elongated, rough, black swellings on twigs, branches, and sometimes even the trunk. The knots are a velvety olive green in the spring. They gradually become hard, brittle, and coal black, as in the image.
Black knot looks much like crown gall (a bacterial disease), but swellings from black knot are located on branches rather than on roots and at the crown. Additionally, crown gall never turns black. Injury from black knot may be fairly severe. If stems become girdled, dieback is evident. The trees gradually weaken and may die unless effective control measures are taken. The causal fungus, Dibotryon morbosum, can infect at least two dozen species of cherries, plums, and other members of the Prunus genus, including some ornamental species.
Purchase only disease-free nursery stock. Never buy trees with visible knots or abnormal swellings on the twigs and branches. Look for this disease in its early stages, appearing as light brown swellings that later rupture the bark and turn darker. Prune and burn (or bury) all infected wood in late winter or early spring before growth starts and as soon as new knots appear. Make cuts 4 to 8 inches behind any obvious, black-knot swellings. Knots on the trunk or on large limbs should be carefully cut out with a knife and chisel, removing about an inch of healthy bark and woody tissue beyond any visible gall tissue. If possible, destroy (burn) all available wild, neglected, or worthless plum and cherry trees.
Most infections occur between budbreak and 2 weeks after bloom, when wet conditions are accompanied by temperatures of 55o to 77oF. If fungicides are used, sprays should be applied as buds open and must be continued every 2 weeks until about 3 weeks after petals fall for effective protection against this fungus. These early-season fungicide sprays do much to prevent new infections but will not stop infections that are already present on the tree. The fungicides are protectants, used in situations where black knot is found in your area. Chemical recommendations for ornamental trees are listed in the Illinois Commercial Landscape & Turf Pest Management Handbook. The only product that we can recommend for homeowners is copper, which comes in many formulations; so read the label carefully to be certain that the formulation you choose is registered on the host tree that you have. It must be labeled for use against the black knot fungus. Besides the chemical use, all visible knots must be pruned from the trees to remove old infections.
For more information concerning this disease, consult Report on Plant Disease (RPD), no. 809, “Black Knot of Plums and Cherries,” available at http://www.ag.uiuc.edu/~vista/abstracts/a809.html.