Black knot

Black knot is a severe fungal disease caused by the pathogen Apiosporina morbosa, which primarily affects stone fruit trees. This disease is notorious for causing distinct, black, knot-like swellings on the branches, which can eventually girdle and kill the infected limbs.

The disease affects a wide range of Prunus species, including plums, cherries, and apricots. The fungus invades the tree's cambium layer, stimulating excessive and disorganized growth of plant tissue. Over time, these formations become hard, charcoal-like, and heavily infected with fungal spores, serving as a persistent source of inoculum.

Key symptoms include the development of olive-green, soft swellings on young branches in the spring. As the season progresses, these swellings darken, harden, and develop a cracked, black, wart-like appearance. Heavily infested trees may exhibit stunted growth, branch dieback, and reduced fruit yield.

The spread of the fungus is highly dependent on environmental conditions, particularly wet and humid spring weather. Ascospores are released from the black knots during periods of rain and are dispersed by wind and splashing water to new, susceptible tissue on both the same and neighboring trees.

Management of Black knot requires an integrated approach to sanitation and chemical control. Key strategies include:

• Pruning out and destroying all infected branches 8 to 10 inches below the visible knot.
• Removing nearby wild or neglected host trees that harbor the fungus.
• Applying protective fungicide sprays during the dormant season and throughout the period of rapid shoot growth.