Black knot is a rather ugly disease, but one that can be controlled with pruning and fungicide applications. 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. The problem is quite common in the northern part of Illinois, but we do not often see it downstate. I don't think this has to do with the natural range of the disease as much as the greater number of intensely landscaped areas near the larger cities.
Black knot causes elongated, rough, girdling, black swellings on twigs, branches, and sometimes even the trunk. The knots are a velvety olive green in the spring when sporulation occurs. They gradually become hard, brittle, and coal black. If stems become girdled, dieback is evident. The trees gradually weaken and may die unless effective control measures are taken.
Purchase only disease-free nursery stock. The older, black knots represent at least two years of growth. Never buy trees with visible knots or abnormal swellings on the twigs and branches. Look for this disease in its early stage: light brown swellings that later rupture the bark and turn darker.
Most infections occur between budbreak and two weeks after bloom if wet conditions are accompanied by temperatures of 55 to 77 degrees F. For effective protection against this fungus, fungicide sprays should be applied as soon as buds open and must be continued every two weeks until about three weeks after petals fall. Early-season fungicide sprays do much to prevent new infections but will not stop infections that are already present--in this case the fungicides are protectants, not cures. The only product we can recommend for homeowners is copper. There are many formulations of copper, so read the label carefully to be sure the formulation you choose is registered for use on your species and for use against the black knot fungus. Registered chemicals are listed in the Illinois Commercial Landscape and Turfgrass Pest Management Handbook, 1998-1999.
Besides spraying with fungicide in the spring, you must prune all visible knots to remove old infections. Do not prune now. In late winter, or early spring before tree growth starts and as soon as new knots appear, prune and burn (or bury) all infected wood. Make cuts four to eight 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, burn all available wild, neglected, or worthless plum and cherry trees.
For more information concerning this disease, consult Report on Plant Diseases No. 809, Black Knot of Plums and Cherries.