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What Is a Fungus on Tree Bark?

Single tree. Autumn. Single Tree in the field. image by Igor Golovnov from

Fungal infections on or under a tree's bark can cause large external diseased areas known as "cankers" to develop. Also known as heart or sap rots, fungal bark infections destroy the internal supportive cell components and the external bark of the tree. As the fungus advances, the tree becomes unable to support its own weight. Branches that are infected might drop to the ground and, in severe cases, the entire tree might tumble down.


Fungi that infect the bark and wood of trees can be identified by the form of canker or growth that the tree develops. The canker, conk or bracket might form where the fungus first entered the tree. This often happens where the tree's bark was once weakened, such as where branches were removed or where the trunk sustained damage through over-cultivation.


Most bark and wood fungi are classified as white, brown or soft rots. When white rot fungi infects a tree, the wood becomes soft to the touch. The area of infection appears yellow or white and exudes moisture. Brown rot fungi causes the bark to become dry and crumbly. The area might sustain long cracks. Soft rot fungi tends to occur on trees that have already fallen to the ground and died. They are nature’s way of naturally breaking down the wood on the forest floor.


Fungal infections of a tree are usually caused when the tree sustains injury. Most trees can quickly recover from an invasion of fungus, but if the tree is stressed, it is more susceptible to serious injury. Drought, insect infestation and the overuse of herbicides can all weaken the tree and cause bark canker fungal infections to occur, according to the North Dakota State University.


The spread of fungal bark rots occurs when the fungus grows just beneath the bark's surface. During wet weather, such as spring rains, the fungus produces spores, according to Iowa State University. It releases spores into the air through the cracks or cankers in the tree's bark. The wind picks up spores. When the spores land on damaged areas of a neighboring tree, they quickly invade.


Prune away any areas of the tree that suffer from cankers. Avoid pruning during wet weather since this can cause the fungi to release excessive amounts of spores into the air. If the cankers are large and occur on the tree's bark, the tree might require removal. Improving the tree's overall health through adequate water, pruning practice and fertilizer applications will often help the tree fight the fungus on its own. Many times, the areas of canker on the tree's bark will be shed off like a scab that has healed as the tree regains heath and is able to combat the fungus on its own.

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