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Black Knot Disease 

Black knot, caused by the fungal pathogen Apiosporina morbosa, is a common disease in Montana. The black galls/knots are very noticable on branches during fall and winter. 

Hosts: Cherry, chokecherry, plum, and other ornamental Prunus species

Damage/Symptoms: This fungal disease is characterized by the development of thick, black, gall-like swellings on twigs and branches. The galls can girdle the branches of highly susceptible trees, causing leaf wilt and branch dieback beyond the galls.

Figure 1: Tree with multiple black knot galls. Photo by Sarah Eilers, Montana State University.

Disease Cycle: The fungus overwinters in the black, knobby galls (Fig. 1). During wet periods in the spring, spores are released and infect young green shoots or wounded branches. New galls develop during the summer and early fall and may be covered in olive-green spores in the following spring (Fig. 2). The new galls turn black and hard two years after the initial infection. 

Figure 2: Black knot gall developing on young shoot. Photo by Paul A. Mistretta, USDA Forest service,

Management: Black knot can be managed by pruning out existing galls in late winter. Make the pruning cut at least 4 inches below the infection. Disinfect pruning tools between cuts with 70% ethyl alcohol or a standard household disinfectant spray. Dispose of the galls to reduce inoculum levels. Fungicides can be applied in spring to protect young or highly susceptible trees. Fungicide treatments must be applied when flower buds are beginning to open and may have to be repeated depending on the product used, label instructions, and weather conditions. Fungicides with one of the following active ingredients have shown good control of black knot: chlorothalonil or thiophanate-methyl. 

Disclaimer: These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. The authors and Montana State University assume no liability resulting from the use of these recommendations. The Montana State University Extension Service is an ADA/EO/AA/Veteran's Preference Employer and Provider of Educational Outreach.

Eva Grimme
[email protected] 

MSU Extension
Montana State University
P.O. Box 172230
Bozeman, MT 59717-223

Tel: (406) 994-1750
Fax: (406) 994-1756
Location: Culberston Hall
[email protected]