Black knot is a serious disease of plum and cherry trees (Prunus species) throughout the United States. Black knot is a disease that gets progressively worse each year unless controlled, and it will eventually stunt or kill the tree. It is frequently seen in the woods on wild black cherry (Prunus serotina) and in orchards that are not sprayed regularly. Hosts include American, European, and Japanese cultivars of plums. Although wild black cherry is a very common host, damage is less severe on sweet and sour cherries. Apricots, peaches, and flowering almonds are occasionally damaged.
The most obvious signs of the disease are the hard, black, swollen galls, commonly called knots, on branches and twigs. Black knot also infects fruit spurs, and sometimes trunks. Infected trees may produce few flowers or fruit. Usually infection originates in the newest growth, causing small twigs to die. The conspicuous black gall does not appear until the second year of infection. As the knots grow they eventually cut off the flow of water and nutrients to the branches, causing stunting, wilting, and dieback. Gradually the entire tree may weaken and die if the severity of the disease increases.
Black knot is caused by the fungus Dibotryon morbosum (formerly called Apiosporina morbosa). Spores are released from mature knots from early spring to early summer, and carried by wind and rain. The fungus enters the plant, usually on the youngest growth, either through wounds or by penetrating the bark. Most infections occur under wet conditions when the temperature is between 55 and 77 degrees F. Plants are most susceptible when they are blooming. By autumn, light-brown swellings appear on twigs, which eventually rupture as they enlarge. The following spring, the rapidly growing knots are covered with a velvety, olive-green fungal growth. They become larger and darker over the summer. By fall, they are hard, rough, and black. Slowly the knots enlarge and girdle the twig or branch, killing it. It may take the twigs or branches several years to girdle and die.
- Plant resistant species. Japanese plum varieties are less susceptible than most American species. Purchase disease-resistant trees that are free of abnormal swellings or visible knots.
- Pruning is the most important control measure. Research has shown pruning can reduce infection by 80%. Prune off all knots in late winter or early spring before growth starts. Bury, burn, or compost all discarded plant material. Pruning cuts should be made at least four to eight inches below any swellings or knots because the fungus grows beyond the edge of the knot. Pruning tools should be sterilized between cuts using 1 part bleach to 4 parts water.
Very few fungicides are available for this disease and they are often not very effective.
The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.