Black knot

Black knot, caused by the fungus Apiosporina morbosa, is a destructive plant disease that primarily targets trees and shrubs within the Prunus genus. This fungal pathogen manifests as persistent, dark, and irregular swellings on twigs and branches, effectively girdling them and preventing nutrient transport.

The disease is particularly devastating to orchard favorites, including sour cherry (Prunus cerasus) and its varieties like the Duke cherry (Dura cherry), as well as various plum cultivars. Once established in an orchard, the fungus can persist for years if not managed through rigorous pruning and sanitation practices.

Symptoms begin as small, light-brown swellings on current-season shoots, which eventually enlarge and develop into hard, rough, black knots. These galls often harbor the fungus throughout the dormant season, releasing spores during the wet spring months to initiate new cycles of infection on tender, growing tissue.

High relative humidity and frequent rainfall in early spring are the primary catalysts for spore dispersal and colonization. Furthermore, the disease is frequently spread through the movement of contaminated nursery stock or the use of infested pruning tools, making sanitation protocols essential for any commercial or backyard orchard.

Managing Black knot requires a multi-faceted approach, emphasizing prevention and early intervention. Pruning out infected branches during the dormant period is critical; removed material should be burned or buried to prevent further sporulation. Following pruning, the application of protective fungicides can help safeguard remaining healthy tissue during the critical period of spring growth.