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Black knot

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Disease name: Black knot

Pathogen name: Apiosporina morbosa (syn. Dibotryon morbosum) (fungus)

Hosts:  Prunus species (plum and cherry trees are most susceptible; peach, nectarine and apricot are less commonly infected).

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 managed.  Over time it may 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. Although wild black cherry is commonly affected, damage is less severe on sweet and sour cherries.

The most obvious sign of black knot is 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. In the first year of infection, the infected stem will swell and the bark may begin to split. 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 even die if the severity of the disease increases.

Black knot overwinters as fungal spores in the knots. Spores are released from mature knots from early spring to early summer, and are 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. 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 die.

Cultural Management:

Plant species that do not belong to the genus Prunus. If a species in that genus is desired in the landscape, the Japanese plum varieties are less susceptible than most American species, and peach and nectarine are less commonly infected. Purchase trees that are free of abnormal swellings or visible knots.

Pruning is an 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. Pruning is best done in the early stages of infection. Pruning a heavily infected tree with numerous knots could lead to an aesthetically undesirable landscape tree.

Chemical management: 

A limited number of fungicides are available for black knot and they are often not effective.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. 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.

For current pesticide recommendations, contact The Morton Arboretum Plant Clinic (630-719-2424 or