Black knot is a disease that attacks American, European and Japanese varieties of cultivated plum and prune trees as well as wild cherry. Cultivated sweet and sour cherry trees are rarely affected. It is found in commercial and home orchards and on ornamental plum trees. The disease is caused by the fungus, Dibotryon morbosum, and it strikes woody parts of the tree including the twigs, limbs and, sometimes, the entire trunk.

The common name, "black knot" describes the main symptom of the disease. The initial symptoms are small, light brown swellings at the base of leaves, on fruit spurs, and on terminal shoots. These appear during the first year after infection occurs. These growths enlarge and develop an olive green growth over their surface. The green color eventually disappears and the knots become coal black and hard. The knots vary in size, location and shape but they often coalesce into large knots that cover whole sections of the branch.

Life Cycle - Black knot infections are initiated by spores which germinate and infect the current year's growth. Infection can occur from green cluster stage of bud development until tree growth ceases in June. After infection, a light brown swelling develops during the same year or the following spring. The swelling turns olive green and produces spores within one or two years after infection.

Plant ornamental trees that are not susceptible to black knot.


Since the knots are localized in nature, the disease can often be controlled by cutting off twigs and branches 4 inches or more below the last visible sign of the knot. This is generally most effective if done during the dormant period (winter). On large, main branches or trunks, knots should be cut out with a sharp knife or chisel. One inch of healthy bark around the knot should be included in these cuts. Taper the cuts to a point on each end to promote proper closing of the wound. Infected parts removed from the trees should be destroyed away from the tree.

Spores can develop and spread from parts left on the ground or in brush piles nearby. Application of a fungicide labeled for black knot control may help prevent further spread. The fungicide would have to be applied in early spring before the buds open to be effective. Two additional sprays at 7 to 10 day intervals are generally also recommended. When planting fruiting or ornamental plum trees, try to obtain varieties resistant to black knot.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.


 

Some fruiting and ornamental plum cultivars:

  • Very Susceptable: Bluefre, Damson, Shropshire, Stanley.

  • Moderately Resistant: Bradshaw, Early Italian, Fellenberg, Formosa, Methley, Milton, Santa Rosa, Shiro

  • Resistant: President

 

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