Be alert for black knot disease
Trees and shrubs in the genus Prunus, which includes cherries and plums, are susceptible to a fungal disease commonly called “black knot” for the thickened, knobby growths or galls that appear on twigs and branches. The fungus (Apiosporina morbosa) that causes black knot affects wild and cultivated species and can be very destructive if left unmanaged.
As in other fungal diseases, black knot is reliant on its spores being spread by wind and rain. In spring, spores released by existing galls find their way to new, soft twigs.
During the first season, black knot is hardly noticeable, appearing only as a slight swelling late in the year or following spring.
In the second season, black knot appears as an olive-green or reddish-brown swelling on soft twigs that will eventually rupture and become hard, black knobby in appearance.
By the third year, it can produce and release spores during wet periods in spring perpetuating the disease and its spread.
The fungal knots can grow quickly, creeping along branches and affecting more of the tree. After several years, black knot left untreated may deform branches, causing them to bend or grow oddly. Branches may die after becoming completely encircled by the fungus galls.
Even the trunk of a cherry or plum tree may be affected by black knot, resulting in very large galls. Black knot on tree trunks can hasten the death of the affected tree by causing cracks in the trunks creating an entry for other fungi to infiltrate and initiate rot.
So, what can be done when black rot visits your cherry or plum? There are several management tools that may be used.
Pruning is the easiest and can be effective if the black knot fungus has not spread significantly. Pruning should be done in winter or in early spring while the temperature is below freezing and before any new growth has begun. Pruning should be at least 4 to 6 inches below the knot areas.
For large branches and tree trunks black knots can also be cut out. Remove the diseased wood and about 1 inch of clean wood around the knot. If necessary, remove the entire branch.
Because the knots can still produce spores even after cutting from the tree, pruned knots and branches should be placed in a trash bag and disposed of. Pruning implements should be disinfected between each pruning cut with a solution of 10% bleach or 70% alcohol to prevent reinoculation of the tree at the next pruning site.
Fungicides containing sulfur, copper, chlorothalonil, captan or thiophanate-methyl can be applied to trees and shrubs that are not heavily infected with black knot after the galls have been removed.
Follow all of the directions on the fungicide label and be sure that the plant you are treating is listed there, as not all fungicides registered for use on ornamental Prunus varieties can be used on edible ones.
Fungicides should be used in the spring, beginning when the flower buds are just beginning to open. Repeat applications according to the label directions until new growth at the branch tips has ended or the weather has become consistently warmer than 60 degrees. Fungicide application should be timed prior to any rain event when spores are most likely to be spread to a new site.
For heavily infected trees, removal of the entire tree is the best option since the cost of multiple fungicide treatments and pruning for a succession of years may become cost prohibitive.
There are some cultivars of plum that appear to have some resistance to black knot including: President, Methley, Milton, Early Italian, Brodshow, Fellenberg, Shiro, Santa Rosa and Formosa.
There are no cultivars of cherry trees that show resistance to black knot, though it is thought that the sour cherry cultivar, Evans Bali Cherry (Prunus cerasus “Evans Bali”) may have some resistance.
Do not plant new trees in the same area where infected trees were removed. Never purchase new trees that may exhibit symptoms of black knot.
To learn more about black knot disease, visit Penn State Extension (https://extension.psu.edu/black-knot-of-prunus-in-the-home-landscape). If you have questions on black knot fungus or any other questions, call the Master Gardeners of Butler County Garden Hotline at 724-287-4761, ext. 7, or email the Master Gardeners at butlermg@psu.edu.
Mary Alice Koeneke is a Penn State Extension Butler County Master Gardener.
