Black Knot Noted on Area Cherry Trees

Black Knot Noted on Area Cherry Trees

Black Knot Notes on Area Cherry Trees, Nebraska Extension Acreage Insights - June 2017, http://acreage.unl.edu/enews-june-2017
Fig. 1. Black knot on cherry tree branch. Photo by Ginny Koranda, Seward County Extension office.

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).

Fig. 2. Spring growth of black knot. Note the velvety green fungal growth in the upper right hand quadrant. Photo by Ginny Koranda, Seward County Extension office.

Disease Cycle
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.

Michael Rethwisch
Michael Rethwisch
Extension Educator - Crop Productivity

Michael is an Extension Educator in Agronomy for Polk and Bulter Counties. He has his master's degree in entomology, and currently serves as the superintendent for the Nebraska State 4-H Weeds and Grass Identification contest. He also serves as the coach for the Nebraska 4-H horticulture team. with the team placing 2nd in the national 4-H Horticulture Contest in 2014.

Contact Michael at:
451 N 5th Street
David City, NE
68632-1666
402-367-7410