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  • Black knot infections occur in springtime during the active phase of shoot growth when rainfall favors the spread of this fungus. Wild cherry and plum serve as alternate hosts, spreading the disease to cultivated plums and making chemical control more difficult in susceptible varieties.
  • This disease is common in plum orchards. Symptoms appear in new shoots as green swellings of the stem and oddly shaped growths covering the shoot that eventually turn black with warty surface.  In severe cases, the fungus girdles the branch.  Cultivated plums vary in susceptibility with Stanley and Methely being highly susceptible, and Obilinaja fairly resistant.
  • The most important period for black knot sprays is from white bud to shuck split.  Infections require rain and are favored by temperatures above 55 F.

Disease Cycle and Management

Black knot is caused by the fungus Dibotyron morbosum, also known as Apiosporina morbosa.  It overwinters on infected trees, and is common on wild cherry and wild plum, as well as, ornamental stone fruit species.  Dispersal occurs from ascospores released during rain beginning soon after budbreak at the green cluster stage and continue throughout shoot elongation which continues into summer, but most occur just before and during petal fall.  Symptoms become visible in fall.  Initially, infections appear as green swellings on new shoots, but they eventually turn black in their second year.  Elongated swellings that can vary in length from 1 to 12 inches. Because of their green color, it is easy to miss infections in their first year.  The knot continues to develop through the second summer with ascospores maturing the second winter after infection. 

Infections will affect the woody part of the tree, so with severe infestation, there is a yield reduction. Once established in an orchard, it is difficult to control solely by chemical means. Well-time fungicide sprays can prevent infection. However, where wild infected hosts are near the orchard, complete control is not possible in susceptible varieties.  In this case, infections should be removed during dormant pruning by cutting back several inches into healthy tissue.

Activity Rating of Fungicides

  • Copper (Cuprafix, etc.) - slight
  • Captan - good
  • Chlorothalanil (Bravo) - excellent
  • Sulfur (Microthiol Disperss, etc.) – none (ineffective)
  • Thiophanate-methyl (Topsin M) - fair

Nonchemical control is primarily by removing wild plum and cherry, and by removing infections during pruning.  Where this is not possible, select varieties with partial to complete resistance:

Spring Satin plumcot, Obilinaja, Early Italian, Green Gage, Fellenberg, and President.

Black knot infection of plum shoot in an early stage

Black Knot Infection
black knot on plum branch

Black Knot