Black knot is common in the north-central and north-eastern states. In those areas, it is a common problem on ornamental Prunus species and on edible plums. Hosts include ornamental plums and cherries, often planted for their flower and foliage color, as well as ornamental bark characteristics. About 24 species of Prunus are susceptible to this disease. Flowering almond, apricot, blackthorn, cherries (bird, bitter, black, mahaleb, Nanking, pin, sand, western sand, sour, and sweet), chokecherry, peach, and plums (American, beach, Canada, common, Damson, Japanese, myrobalan, and Siberian) are included in this list of possible hosts.
The disease is usually noticed in the spring when leaves are not fully expanded and galls are more obvious. This disease is caused by a fungus, Apiosporina morbosa, which infects new twigs in the spring. There is a slight swelling of the infection site by fall, but it usually goes unnoticed. The following spring (1 year after infection), the swellings continue to grow and become roughened. These 1-year-old galls are visible now. The galls will become elongated, rough, black swellings on the 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 dies. Because black knot galls are perennial, they continue to spread in the branch. You will see larger galls next year if they remain on the tree. The disease does not typically kill a tree but causes deformed growth if left unchecked.
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 your trees have this disease, take steps to get it under control using a combination of pruning and fungicide applications. Mark your calendar to prune in the dormant season when galls are easy to see. Remove all knots from the tree and burn, bury, or remove them from the site. The fungus can release spores from the dead knots. Make cuts 4 to 8 inches behind any obvious black knot swellings. 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 a 10% clorox solution between cuts and between trees.
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 wild trees should be removed.
Most infections occur between budbreak and 2 weeks after bloom when wet conditions are accompanied by temperatures of 55° to 77°F. Infection occurs through new shoots and wounds. 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. Products are listed in the Home, Yard, and Garden Pest Guide and the Commercial Landscape and Turfgrass Pest Management Handbook. Remember that early season fungicide sprays prevent new infections but do not stop infections that are already present, thus the pruning recommendation. Fungicide use is usually reserved for edible plums and is used in conjunction with pruning. For more information, consult Report on Plant Disease, no. 809, “Black Knot of Plums and Cherries,” available on Extension’s VISTA Web site.