Black Knot Fungus

Black fungus on a tree can look like anything from a light dusting of soot to hard, charcoal-like lumps. Identifying which one you have is key because some are harmless, while others can kill the tree. 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.

• Target Trees: Primarily fruit and ornamental trees in the Prunus genus (Cherry, Plum, Apricot, Chokecherry).
• Danger Level: High. It "strangles" branches, preventing nutrients from flowing. If it reaches the main trunk, it can kill the tree.
• Treatment: Prune out the infected branches during the winter (dormant season). Cut at least 4–6 inches below the knot into healthy wood. Sterilize your tools with alcohol or bleach between every single cut to avoid spreading it.

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.