Black knot is a common problem on ornamental Prunus species, as well as on edible plums. Hosts include the ornamental plums and cherries that are often planted for their flower and foliage color. The disease is usually noticed first in the spring when leaves are not fully expanded and galls are more obvious. Martha Smith of the Macomb Extension center reports many cases of black knot this year in the northwestern part of Illinois. The Morton Arboretum in Lisle also reports confirmed cases of black knot.
Black knot is caused by a fungus called Dibotryon morbosum, which infects new twigs in the spring. There is a slight swelling of the infection site by fall, but it will probably go unnoticed. The following spring (1 year after infection) the swellings continue to grow and become roughened. These 1-year-old galls are probably what we are seeing now. Black knot is named for the elongated, rough, girdling, black swellings on twigs, branches, and sometimes the trunk. The knots become hard, brittle, and coal black. If growth of the fungus is on one side of the stem, the stem may be bent at the knot. If the knot girdles the stem, the stem beyond will die. Because black knot galls are perennial, they will continue to spread in the branch. You will see larger galls next year if they remain on the tree.
When buying ornamental Prunus species, inspect stems carefully for galls and swellings that may indicate early black knot infection. The older black knots represent at least 2 years of growth. Never buy trees with visible knots.
If you should find that you have this disease in your trees, take steps to get it under control using a combination of pruning and fungicide applications. Mark your calendar to prune in February. Remove all knots from the tree and burn, bury, or remove them from the site. Make cuts 4 to 8 inches behind any obvious, black knot swellings. Actually, you can prune any time the tree is dormant; but if you wait until February, you will be able to see all of the knots. Apply a dormant oil at bud swell. You could still prune now, but sap will flow freely from the cut surfaces, attracting many insects. Regardless of the time of year, prune only in dry weather and take the time to disinfect pruners with rubbing alcohol or 10% bleach solution.
Wild plums and cherries are more susceptible to black knot than cultivated varieties. If your landscaped area is near a wooded site, look for galls on the wild Prunus species. Infected trees should be removed.
Most infections occur between budbreak and 2 weeks after bloom when wet conditions are accompanied by temperatures of 55 degressF to 77 degreesF. For effective protection against this fungus, fungicide sprays should be applied as soon as buds open and must be continued every 2 weeks until about 3 weeks after petals fall. Many copper fungicides are registered for use against black knot, so pick a formulation that you prefer, being careful to read the label for host and disease clearance. Remember that early season fungicide sprays will prevent new infections but will not stop infections that are already present, thus the pruning recommendation. Fungicide use is usually reserved for edible plums and is made in conjunction with pruning. For more information concerning this disease, consult Report on Plant Diseases No. 809, Black Knot of Plums and Cherries.