Have you ever been walking in a natural area and seen something black and lumpy on the branches of shrubs? You might have wondered, “Who would put scat on a stick?” It may surprise you to know that this is actually not scat, but Black knot!
Black knot, known scientifically as Apiosporina morbosa, is a common fungal disease that affects fruit trees. Ornamental, plum and cherry trees, both wild and cultivated, are heavily affected but many types of fruit trees can be affected. Mayday Trees, also known as the Amur Cherry, are a popular ornamental tree in Edmonton and can be affected by black knot fungus. This fungal disease presents as swollen, hard black galls (tumor-like growths) that often look like scat on a stick. Galls typically form on the branches of affected trees, however, they can present on the trunks as well as either swollen or blackened sections. Galls will be most noticeable in fall and winter months, once leaves have fallen. This fungus is capable of surviving cold winter months, overwintering within the galls protected from the elements.
Black knot fungus on a branch, two to three years post infection. Photo by EALT.
What causes Black Knot?
Wet periods of springtime rain release fungus spores from infected trees; these spores are carried by wind and can attach to unhealthy or new-growth branches. Wild or neglected trees are common causes of black knot outbreaks, spreading the fungus to even the most well-maintained of trees. Black knot grows inside the tree, branch or trunk, for several months before the trees show outward signs of infection. Like a tumour, as the fungus develops, it releases chemicals that cause the host tree to grow additional plant cells unnecessary to the tree development. These extra cells growth creates visible, swollen wood galls. The trademark black knot fungal galls consist of both fungus and tree cells. Within one year of infection, the gall has an olive-green fungal growth look. Within two to three years of infection, these galls will become hardened and black in appearance. Once galls reach this stage, they are capable of releasing spores.
Black knot fungus on a branch in autumn. Photo by EALT.
Does Black Knot harm the tree?
There are many Prunus trees that are tolerant of black knot, they can withstand multiple galls with little negative effect on the tree's health. However, some trees can be affected more severely, their shoots and leaves wilting and dying on gall-infected branches. The leaves of black knot-tolerant trees’, even on affected branches, will remain green and healthy. Leaves on trees which are more susceptible to the fungus will wilt, turn brown and die on gall-affected branches; making these trees more noticeable. If the branch dies, the galls will die with it; this sometimes occurs after spores are released in early spring. However, if the branch survives, the black knot gall will increase in size and continue to produce spores every spring. If the gall becomes bigger than the supporting branch and encircles it, this is known as girdling the branch. If the branch becomes girdled, the branch and leaves beyond it will die. Galls growing on the tree trunk will not rot it, however, it does create cracks that potentially leave the trunk open to infection from other wood-rotting fungi.
My tree is infected, what should I do?
Volunteers removing Black Knot at Pipestone Creek. Pruning Black knot should only take place in the winter months, ideally in temperatures below 0 degrees.
If you notice black knot fungus within your own trees, avoid planting other Prunus trees until the infection is dealt with. Infected trees can be pruned or removed entirely to reduce the spread of black knot fungal spores. Inspect trees and shrubs prior to purchasing them from garden centres. Research trees prior to planting, consider choosing species that are more tolerant of black knot; these trees will withstand infection and subsequent pruning to remove affected branches and galls.
Galls can be removed to enhance the look of trees as well as reduce the amount of spores produced in spring-time. Pruning should occur at least 15 to 20 centimetres below the gall and infected branches should be burned or properly disposed of. Cuts should be made in late winter when temperatures are below 0 degrees Celsius to prevent black knot from re-infecting the pruning efforts.
EALT staff spend a lot of time removing Black knot in winter months.
Black Knot and EALT
Black knot fungus is something that affects our local forests and if untreated, it can to the eventual death of infected plants. EALT works hard to monitor infections of Black knot on our conservation lands, and Black knot removal is our primary fieldwork priority in the winter months.
Even though black knot is a native part of our ecosystems, it has a disproportionate effect on urban forests an ornamental trees, as woodlands and forests become less prominent on the landscape. Deregulated seasonal cycles due to climate change have also had an effect, providing atypical warm spells in the middle of winter and ample moisture for the fungus to spread more rapidly.
If you’d like to help us with our hands-on stewardship efforts to minimize the presence of Black knot on our lands, you can do so by looking on our Events page in the winter months. We’d be very pleased to have you along!
