Black knot

Black knot, caused by the fungus Apiosporina morbosa, is a common and destructive disease affecting members of the Prunus genus. It is particularly prevalent on plum, cherry, and apricot trees, resulting in the development of unsightly, hard, black galls on the branches and twigs of infected plants.

The symptoms begin as small, greenish-brown swellings on the surface of young stems. As the infection progresses over the following season, these growths enlarge, darken, and become hard, cracked, and coal-black in color. These knots can range from a few centimeters to over a foot in length, significantly distorting the tree's architecture.

The life cycle of Apiosporina morbosa relies on the production of ascospores. In the spring, during wet weather and bloom time, these spores are released and dispersed by wind and splashing rain to new, succulent green shoots. The fungus then penetrates the host tissue, establishing a permanent infection that continues to grow throughout the year.

Conditions favoring the spread of the pathogen include prolonged periods of spring moisture and cool to moderate temperatures. Dense, unpruned tree canopies create microclimates that retain moisture, further facilitating spore germination and infection, especially in orchards located near wild or neglected host trees.

The impact of black knot is severe, as the galls eventually girdle the branches, restricting the flow of water and nutrients. This leads to flagging, premature branch death, and in cases of heavy infestation, the decline of the entire tree. Management strategies include:

• Pruning out and burning infected branches at least 4-8 inches below the knot.
• Maintaining tree vigor through proper fertilization and watering.
• Applying copper-based fungicidal sprays during the dormant season to protect new growth.