Black Knot Fungus

The black death of trees, the black knot is as ghastly as it is apparent.

The fungus Apiosporina morbosa causes black knot. Black knot galls are most conspicuous during the fall and winter after all the leaves have fallen. So before the first buds of spring emerge, give your cherry trees a quick scan.

Even an inexperienced gardener can quickly identify black knot by its swollen, knobby black growths. These unsightly black growths emerge along the length on branches of a range of trees common in Minnesota yards. There may be many black galls on trees infected with the black knot.

The black knot fungal disease infects prunus trees, which include ornamental, edible, and native plum and cherry trees. Many Prunus trees tolerate black knot, but others are more severely affected.  In these vulnerable trees, leaves and shoots wilt and die on branches with galls.

The Lifecycle of Black Knot Fungus

During the wet periods of spring, the spores of the fungus get released. The spores are spread to other trees by the wind, where they infect young green shoots or wounded branches. The fungus grows inside the branch for many months with no visible symptoms of the disease.

As the fungus grows, it releases chemicals that make the tree grow extra plant cells that are abnormally large. In this way, the black knot is similar to cancer. This abnormal cell growth results in the swollen, woody galls on trees. What makes these galls particularly interesting are made up of both plant and fungal tissue. Black knot galls on trunks are often cracked and may ooze sticky liquid. Wood decay fungi may enter the trunk through cracks caused by black knot galls and cause wood rot.

Sometimes, the branch and the gall die after spores are released in early spring. If the branch lives, the knot keeps getting bigger and produces new spores every spring. One year after infection, galls become a swollen area of the branch with a velvety olive green covering of fungal growth. Two years after infection, the gall has turned black and hard. These galls release spores in spring when wet.  

The gall can completely encircle and girdle a branch. When this happens, the leaves beyond the gall wilt and die.

Treating Black Knot

The black knot fungus remains throughout winter in the galls on tree branches and trunks. During this time the growths are easy to spot.  Black knot is one specific tree disease that can benefit from winter pruning and spring treatment.

Winter Pruning

Pruning of infected trees should be done late winter (February or March) when the temperatures are below freezing. This important because it prevents black knot spores from infecting the pruning wound. To keep your trees free of black knot it is to inspect and prune out your trees each winter.

Infected branches need to be removed from the area and burned. It is essential that the infected pruned branches be handled correctly because the cut infected branches continue to release spores that can start new infections in the tree.

Spring Treatment

To protect young shoots from the black knot fungicides must be applied in early spring. Fungicides should be used when flower buds are just starting to open. Because fungicides are toxic chemicals,  professionals should apply them.

Reach out to Birch Tree Care today to book your spring treatment for Black Knot.