Recently a tree branch with an odd looking growth was submitted to an area extension office, and an examination found it to be infected with black knot. This is a fungal disease of cherry and plum trees and occurs throughout the U.S. Black knot is a disease that gets progressively worse each year unless controlled. This disease is fairly easy to detect, with the most obvious signs being the hard, black, swollen galls, commonly called knots, on branches and twigs (Fig. 1).
Black knot fungal spores are released from mature knots from early spring to early summer, carried by wind and rain. The fungus usually enters the plant on the youngest growth, and plants are most susceptible when in bloom. Most infections occur under wet conditions with temperatures between 55-77 F.
By autumn, light-brown swellings appear on twigs, which later rupture. Growing knots are covered with a velvety, olive-green fungal growth (Fig. 2) the next spring. They become larger and darker over the summer.
The conspicuous black gall does not appear until the second year of infection.
Damage and Control
As the knots grow they eventually cut off the flow of water and nutrients to the branches, causing stunting, wilting, and dieback. It may take the twigs or branches several years to girdle and die.
Pruning is the most important control measure, and can reduce infection by 80%. Knots should be pruned in late winter or early spring before growth starts, with cuts made 4-8 inches below any swelling or knots. Pruning tools should be sterilized between cuts. Pruned plant material should be buried, burned, or composted.