Black knot is a gall caused by the fungus Apiosporina morbosa. It most commonly affects cherries, plums and other members of Prunus. A tree is infected by spores in the spring, usually in new growth and sometimes in wounds. The knots first appear in autumn as light brown swellings on the twigs. The galls turn olive-green that summer, and finally hard and black in the winter. The following spring the spores are released and the disease spreads. The cycle from initial infection to release of spores takes two years.
The effects of the fungus are seen as dieback of affected twigs and branches, eventually leading to complete girdling of the branch. The tree is weakened and is therefore more susceptible to further fungal or insect infestations.
To control black knot, one should prune out the growths by cutting at least six inches into unaffected wood. This is done most effectively in early spring before spore dispersal. Sterilize pruning tools between cuts by dipping them in a bleach solution (nine parts water to one part bleach). Remove these prunings from the area and destroy, as the fungus will continue reproducing on the cut wood. Spray tree with Later’s Copper to help prevent recurring infection.
This infection has become so prevalent that it is not generally recommended to plant the Schubert cultivar. There are some potentially more resistant cultivars being evaluated. Check with your nursery.
Dr. Bill Paton and Shauna Peters