Black Knot Fungus Damage of Cherry and Plum Trees

It is fairly easy to spot wild cherry trees in the early spring by the little white pouches or "tents" filled with developing caterpillars in the crooks between branches. A little less obvious but remarkable are black knots and swellings on those same trees caused by Apiosporina morbosa or Black Knot Fungus Disease. In actuality, Black Knot is known to affect some twenty-four prunus tree species, but it is particularly noticeable in cherry, chokecherry, and plum trees.

History and Geographic Range

Initially reported in Massachusetts in 1911, Black Knot Fungus Disease was first described in 1821 by L. D. Schweinitz from Pennsylvania specimens. Since that time, the fungus has spread across North America (United States, Canada, and Mexico). The fungus has been reported in only one instance in Asia, and is not currently found in Europe, according to the American Phytopathological Society.

Spread of Black Knot

During wet periods in springtime (most notably at temperatures above 55 degrees), spores of Apiosporina morbosa infect new shoots and branches. They may also infect damaged bark surfaces. Although the results are not immediately apparent, hormones increase the size¹ and number of infected vascular cells. This is not visually apparent, but growth of the fungus goes on under the bark, until the second year, when swelling, splitting of the bark, and fruiting structures (olive green to buff at first-becoming black later) appear. If the fungus succeeds in encircling the entire branch, stunted growth-even dying back-occurs. This is due to the inability to transport water and nutrients past the damaged area. Branches at first not to seriously affected can worsen with time.

Effective Management

For cultivated species, effective management includes destroying nearby wild prunus trees, which could infect nearby cultivars, planting resistant varieties, removal of fungus fruiting structures, and applying appropriate pesticides. Pruning should include a length of six to eight inches behind the visible infection, using sterilized tools. Pruned wood should be burned or buried. Clorothalonil, or tetrachloroisophthalonitrile, (Bravo) is considered to be most effective in treating Black Knot Fungus Disease. Water and fertilize as needed to avoid stress. In the fall, a lime-sulfur spray may be applied.

A Viable Biological Control?

One biological control hopeful is Trichothecium roseum, which sometimes appears naturally in Black Knot infected trees, and is noticed as pinkish tinge in some of the knots. Curiously, Trichothecium roseum produces a chemical called a mycotoxin, trichothecene, which is toxic to humans. Could this substance serve as a controlling agent for Black Knot Fungus Disease?

¹ Note the impressive cellular changes shown in Figure 3, associated with this article.

Resources and References:

Cornell University - Black Knot Apiosporina morbosa

West Virginia University - Kearneysville Plant Disease Fact Sheet - Black Knot Apiosporina morbosa

Hilton Pond Center for Piedmont Natural History - Black Cherry Knot

University of Massachusetts - Fact Sheet - Black Knot of Prunus

Cornell University Cooperative Extension - Black Knot Biology and Cultural & Pesticide Application Notes