Apiosporina morbosa

Agricultural pest, widespread

Year-round; sporulation in wet weather

American plum (Prunus americana), black cherry (P. serotina var. serotina), and other Prunus species.

Black Knot is a symptom, not the form, of a parasitic fungus, Apiosporina morbosa, infecting living plants of the Prunus genus.

Knots (galls) are black, irregular, bumpy swellings clasping and sometimes surrounding the twigs and branches, sometimes the trunk, of cherry and plum trees and shrubs. They are elongated, furrowed, cracked, ½″ to 12″ long, and ⅜″ to 1″ in diameter.

Spores are produced during extended rainfalls of six hours or more when the temperature is between 60° and 80°F. They are spread by wind and by rain splashing. Where spores land on current season’s growth or wounded tissue an infection occurs. Growth the first year is very slow. The infection first appears in the late summer as a small, warty, greenish-brown or light brown swelling. The fungus overwinters in this state. The following spring the swelling turns olive green and velvety. Over this growing season (the second year of infection) the knot grows rapidly and has a corky texture. Several knots may merge into a single large knot. Eventually, the knot turns black, hard, and brittle. The Black Knot is usually the first symptom of the disease that is noticed. Tissue toward the center of the knot often dies after two years and is invaded by boring insects. The dead gall tissue is colonized by another fungal parasite, Trichothecium roseum, that gives the knot a whitish or pinkish appearance.

Young infected twigs die the first year of infection. Older branches may last several years. Infections cause the host to lose vigor and may eventually be fatal.

Sources: 7.