Black Knot Apiosporina morbosum

Photo and Narrative by Curtis E. Swift, Ph.D.
Colorado State University Extension, Tri River Area

Introduction:

The term `Black Knot' is used for several different disease problems. The more common use of the term, however, relates to the problem indicated in the photograph of the Chokecherry. This `Black Knot' is caused by the fungus Apiosporina (syn. Dibotryon & Plowrightia) morbosum and can result in the death or distortion of trees it infects.

This fungal problem is more prevalent in areas with wet springs. During these periods, spores are forcibly ejected from fruiting bodies of the fungus, carried by wind and rain to various plants in the area resulting in infections on susceptible hosts. Rain is required for the discharge of these spores with infections occuring from the time buds start to open in the early spring until about two weeks after the tree has finished blooming.

Unlike some fungal pathogens, this organism can infect healthy, unwounded tissue. Infections are more severe with wet conditions and temperatures between 55 and 77 degrees Fahrenheit.

Several months after infection small light brown swellings appear. As the fungus continues to develop within the branch, the bark splits and a velvety olive-green growth develops within the crack. In some cases, knots may not be visible until the following spring.

Spores also can be produced in the summer but these seldom cause problems.

The second year the cankers continue to enlarge, turn black and develop fruiting structures within which spores are produced for the next infection cycle. These fruiting structures are produced on the outside of the knots.

On smaller branches, knots occasionally reach 12 inches (30 cm) in length. If a canker encircles a branch, the branch is girdled and the portion above the canker dies. On Black Cherry, large cankerous swelling two or more feet long can develop on the trunks of large trees.

Susceptible Hosts:

At least two dozen species of cherries, plums and other members of the genus Prunus are reported to be susceptible to Black Knot to include: flowering almond; apricot; blackthorn; cherry - bird, bitter, black, mahaleb, Nanking, pin, sand, western sand, sour and sweet; chokecherry; peach; and plum.

It appears that A. morbosa has several strains. For example, the spores from black knot on chokecherry are reported to be non-infectious on wild plum, and vice versa.

Control:

• Fungicides applied during wet springs will help protect susceptible trees but will not prevent the disease unless combined with a sanitation program involving removing knots from trees and eliminating wild hosts. Captan, chlorothalonil and benzimidazoles applied during active shoot elongation are reported as effective fungicides.
• Cutting out infected tissue in late fall or early spring before bud break reduces the production of spores. Pruning cuts to remove small infected branches should be made two to three inches below the swelling. Knots on trunks or main branches should be removed down to the wood and at least one-half inch beyond the margin of the knot.
  • Knots removed from trees should be burned prior to spring to prevent the production of spores.
• With commercial plum orchards, growers should establish and maintain at least a 600 foot border free of wild hosts. In addition, new plum trees should not be planted next to or downwind from old plantings with black knot.
  Resistant plum varieties should be used where possible. Stanley, Damson, Bluefree and Shropshire are considered the most susceptible with Bradshaw, Fellenburg, Early Italian, Methley, and Milton showing a moderate susceptibility. Formosa, Shiro, and Santa Rosa are slightly susceptible. President has been reported to be highly resistant to this disease.

References Used:

Boyce, J.S. 1961. Forest Pathology, Third Edition. McGraw-Hill book Company, Inc.
Jones, A.L. & T.B. Sutton. 1984. Diseases of Tree Fruits. Extension Service, Michigan State University.
Ogawa, J.M., E.I. Zehr, G.W. Bird, D.F. Ritchie, K. Uriu, J.K. Uyemoto (eds.). 1995. Compendium of Stone Fruit Diseases. The American Phtopathological Society. APS Press.
Sinclair, W.A., W.H. Lyon & W.T. Johnson. 1987. Disease of Trees and Shrubs. Cornell University Press.
Tattar, T.A. 1978. Disease of Shade Trees. Academic Press.