Black Knot DiseaseBlack 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.
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.
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. Sincerely, |
MSU Extension |