Black Knot, is a disease commonly found on plums and Prunus species. The fungus causes the plant to produce unusual galls which eventually grow to girdle twigs and branches, restricting water supply and killing portions of the canopy. It is spread to other plants via wind and water.
Manage the disease by carefully pruning away the aﬀected tissue. Use sterile pruning tools to make a cut roughly 4 to 6 inches past the lowest point of symptoms on the branch. After cutting the branch, sterilize the tools again by dipping them in a 10% bleach solution (1 part bleach, 9 parts water -> e.g. 1/2 cup bleach, 4 & 1/2 cups water). You should also spray a bit of this bleach solution on the cut wound on the tree. Repeat this process until all theblackgrowth has been removed. Burn the branches and do not compost them. Prune during dry weather to prevent the spread of the disease. This can be done in the fall and winter if the fruit is still growing on affected branches. The client may need to repeat this process every winter for the rest of the tree’s life or until symptoms no longer appear.
Black knot of plum and cherry, caused by the fungus Apiosporina morbosa, may be overlooked during the growing season when the leaves are hiding the galls, but this time of year they are hard to miss, especially when they are as abundant as they are on the tree in the photo below.
This is a serious disease of these trees and can eventually kill susceptible varieties. Management options include sanitation, resistant varieties and properly timed fungicides.
Where manageable, prune out all galls during the dormant season and dispose of them off-site, burn or bury them. This is because even removed galls may still produce spores that can cause new infections. Prune 6″ below the visible edge of the gall because the fungus can be invading the wood in that area prior to gall development.
This disease can affect both orchard and ornamental varieties of plum and cherry but some of the tart cherries are less susceptible. Native wild cherries are hosts of the disease and provide a reservoir of inoculum for orchards and ornamentals. It’s helpful to remove those nearby where possible. For new plum plantings (fruiting/orchard), ‘President’ is highly resistant. Moderately resistant options include ‘Methley’, ‘Milton’, ‘Early Italian’, ‘Brodshaw’, ‘Fellenberg’, ‘Shiro’, ‘Santa Rosa’ and ‘Formosa’. ‘Shropshire’ and ‘Stanley’ are considered quite susceptible.
Here’s how disease develops: Infections occur in the spring on new growth from spores produced on the surface of 2+ year old galls. Spores are produced and spread during rainy weather and shoots must remain wet for a period of time for the spores to germinate and initiate an infection. Infections can occur at temperatures of 50°F or higher when water is present for the required period of time. Over the course of the first summer, a small greenish brown swelling develops. By the end of the second summer, the gall or knot becomes hard, rough and black. These galls begin producing spores the following spring. Galls expand in size each year until the branch is girdled (killed all the way around) and then they die. Once a twig or shoot is girdled, the portion beyond the gall can’t get any water or nutrients and dies as a result. Sometimes, larger branches and trunks can become infected, presumably through wounds.
What if you have a susceptible tree and want to prevent this disease? If you know you have a source of infection (hosts with galls nearby, either wild or on a neighboring property) and you’ve had some infections, keep up with the monitoring and pruning, fertilize and water as necessary to prevent stress, and use preventive fungicides, such as lime sulfur during dormancy (organic option) or chlorothalonil or others labeled for this disease. Other than lime sulfur, applications should be made as directed on the label beginning at bud swell and until new terminal growth ceases.