Do you have Black Knot?

By Melanie Mathieson
Gardening Guru

Have you ever wondered what those black, knotty masses are on the branches the tree and shrub species belonging to the Prunus family (cherry and plum family)? It is the fungal disease (Apiosporina morbosa), known as Black Knot, but us foresters call it “#@&% on a Stick” as it resembles dried fecal matter stuck to the branches.
This fungus is specific to the species of the Prunus family and you can find it throughout Northwestern Ontario on both wild and hybridized species of Prunus, which include wild species such as choke, pin and black cherries, Canada plum, and all those nursery species such as edible plum and cherry fruit trees and ornamental flowering trees such as flowering plums, ornamental cherries (Schubert Chokecherry), flowering almond, etc.
The disease is characterized by the presence of warty, black galls which may vary in size from 1/2 inch to more than 1 foot in length. Sometimes it can be hardly noticeable with very small galls on a few branches. Most often these unsightly galls draw the attention from homeowners who want to improve the unsightly appearance of affected landscape trees.
Since moving to Thunder Bay, I have found at least 40 trees in both the urban areas of the city and the bush outside of the city where the entire tree branches are covered completely with very large galls and the branches have become so heavy that they are bent right down. These trees need to be completely removed as the infection has gotten out of control.
Young, infected twigs may die during the first year of infection but larger branches may take several years to display severe damage. As the infection stresses the entire tree/shrub, it starts to decline and the galls become larger and more obvious with each growing season. The stress placed on the tree may also make it susceptible to infections by other pathogens. It is not uncommon for the galls to become infested with borer type insects. The portions of a branch infected by a knot become stunted and often the knots will enlarge enough to girdle a branch, cutting off the flow of nutrients causing the entire branch to die. Trees with multiple infections on many branches will become dwarfed and misshapen which in turn reduces attractiveness and any production of fruit if they are a fruit bearing tree.
Because of the long infection process and disease cycle, this disease is often undetected by home gardeners. The leaves can mask the symptoms until firmly established infections are in place allow the fungal disease-causing agent has been present for quite some time in the infected tree. Infections may take place as much as a year or more prior to the development of these characteristic “knots”, therefore, the swellings are normally not noticed until the winter of the second season of infection. It takes a keen observer to notice the subtle, initial symptoms present during the first season of infection. Once established, it is very difficult to manage the disease. Awareness and strict monitoring of susceptible plants should be a priority for all home gardeners.
The first symptoms appear as small, light brown swellings of the current or previous season’s growth. By the next season, the swellings turn olive-green in color and have a velvety texture. Over this growing season, the knots darken and appear to have a hard, brittle texture. The hard, black knots are the typical symptoms associated with the disease. Knots can vary in size from approximately 0.5 to 12 inches in length and minute measurements to 2 inches in circumference. The infected twigs often appear bent at the tips because of extra cellular growth on one side. Trees with heavy infections may contain numerous knots. Some of the older knots may appear white or pink in color. This discoloration is often seen in late summer and is caused by the fungal parasite, Trichothecium roseum.
The fungus overwinters in the knots. About the time of bud emergence in the spring, the first fungal spores are forcibly discharged from the knot following a period of warm, wet weather. Apparently very short periods of wetness (only a few hours) are enough to prompt spore discharge. Temperatures between 60-80 degrees Fahrenheit are ideal for the dissemination, germination and infection of new plant tissue. The spores are spread by air currents and splashing rain. It is the combination of rainfall and temperature that are the key factors in the release of spores not the duration of the rainfall or wet period.
In order to control the fungus and prevent it from spreading, good tree maintenance and housekeeping measures must be introduced. The main strategy to lower disease incidence is the removal of sources of spores (knotty tissue) so all shoots and branches bearing knots should be pruned out during the winter therefore Prunus species should be monitored on a scheduled basis for possible infections. Pruning should be completed before spore discharge begins in the spring, usually about the time that the buds first break. To be sure that even the unseen fungus is removed, the cut should be made at least 6-8 inches below the knot. Also make sure at this time that you check for diseased trees and shrubs on neighbouring properties. The knots are capable of producing spores for some time after removal from the tree so any pruned material should be packaged up in garbage bags and removed from the site regardless of the time of year the pruning takes place. Any leaves from infected trees/shrubs should also be raked up and removed from the property in the fall.
Don’t let the possibility of Black Knot scare you from enjoying some wonderful trees and shrubs from the Prunus family. Check with local nursery staff to see which species, they carry at their nursery, that have been hybridized to tolerate or resist Black Knot. Examples of some plum varieties that offer resistance: President - high resistance; Methley, Milton, Early Italian, Brodshaw, Fellenberg, Shiro, Santa Rose, and Formosa – medium resistance; Shropshire and Stanley - very susceptible to Black Knot. Gardening Guru Tip: interestingly enough I have had four varieties of edible cherry growing in my yard for at least five years, with no sign of Black Knot. They are Montmorency Cherry, Carmen Jewel Cherry, Evans Cherry and Romeo Cherry. These trees have beautiful spring blossoms, dark green foliage, edible fruit come late summer and so far disease free. Certainly varieties to consider for your yard.