Black Knot

Black knot is a severe fungal disease caused by the pathogen Dibotryon morbosum (synonym Apiosporina morbosa). This disease is well-known for its destructive impact on the health and structural integrity of various woody plants, particularly those belonging to the genus Prunus. It is characterized by the formation of large, woody galls that can severely stunt the growth of infected trees.

The primary host plants include plum, cherry, apricot, and various ornamental chokecherry species. The fungus infects the tree through wounds, such as pruning cuts, cracks in the bark, or insect injuries. Once inside, the pathogen infects the cambium layer, stimulating excessive and disorganized tissue growth which manifests as the iconic black knots.

The disease is easily recognized by the presence of hard, black, rough-textured galls that encircle or cover one side of the twigs and branches. In early spring, these knots may appear olive-green due to the production of asexual spores. As the summer progresses, the galls harden and turn jet-black, becoming brittle and prone to cracking, which further stresses the tree's vascular system.

Environmental conditions play a critical role in the development of black knot. Extended periods of wet, rainy, and moderately warm weather during the early spring are ideal for spore dispersal and infection. Once established, the fungus can produce spores for several years from the same gall, serving as a persistent inoculum source within the orchard.

The harm caused by black knot includes the death of twigs and branches, leading to a loss of vigor and eventual decline of the entire tree. If left unmanaged, the infection can spread throughout the canopy, rendering the tree unproductive. Effective management and prevention strategies include:

• Pruning out all infected branches at least 4 to 8 inches below the visible knot.
• Sanitizing pruning tools with a bleach solution between cuts.
• Removing and burning or burying infected material promptly.
• Applying protective copper-based fungicides during the delayed-dormant stage of bud break.