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Fungi Sprouting on Trees Have a Scary Story to Tell

Michelle Grabowski, University of Minnesota Extension Educator

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Photo 1 (left): Basidocarp of Climacodon septenrionalis. Michelle Grabowski.
With October's frequent rain, many gardeners have noticed fungi sprouting from some landscape trees. These fungi may be any number of shapes, sizes and colors. They may arise from the trunk itself, from the root flare of the tree or from the roots. In all cases, fungi growing directly on a live tree tell the tale of heart rot within.

What's in the Trunk?

In order to understand heart rot, gardeners must understand a little bit about the wood within a tree trunk. Trees can have several different kinds of wood within their trunk. Sapwood is composed of living cells with a number of jobs to do. Sapwood cells conduct sap through the tree, store extra energy, close off wounds, and actively fight invading microorganism. In all trees, sapwood occurs in the outer most rings of the tree. Some trees, like maple, birch, beech and poplar form only sapwood. Other trees also form a second type of wood, known as heartwood at the core of their trunk. Heartwood cells are dead cells that serve primarily to add structural support to the tree. Heartwood cells contain a number of toxic chemicals that protect the heartwood from wood decay fungi. How well these chemicals protect the heartwood varies from tree to tree. Trees like cedar and redwoods are so effective at defending against wood rotting fungi that their wood is highly valued for use in wood products like lumber.

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Photo 2 (above): Internal decay shown on cut trunk.

Heart rot can cause decay in both heartwood and sapwood. Many different fungi from the phyla Basidiomycota, can cause heart rot. These fungi are often seen on rotting logs or dead trees as well as on living trees. The mushrooms, shelf fungi and other interesting fungal structures that emerge in wet weather are spore producing structures of the fungus, and are generally known as basidiocarps. If a basidiocarp is observed on a live tree, it indicates that there is rot within the tree. Basidiocarps can be used to identify the specific fungi causing rot as they are often unique to a specific genera or species of fungus.

How the Invasion Begins

Heart rot fungi are not aggressive pathogens and are unable to infect a tree through intact bark. Instead heart rot fungi take advantage of wounds from lawn mowers, weed whips, fire scars, deer rubbing, rodent chewing, frost cracks, broken branches and other injuries to access the sapwood and heart wood. Some heart rot fungi can also enter a tree through an old branch stub or through the trees roots. Once inside the tree, the fungi use a variety of enzymes and other chemicals to breakdown the wood for food.

The Progress of Decay

White rot fungi breakdown all components of the wood including lignin, cellulose and hemicellulose. Trees suffering from white rot have soft white wood that is stringy, spongy, or crumbly to the touch. Brown rot fungi break down cellulose in the wood but not lignin. Trees suffering from brown rot will have brown decaying wood that breaks apart into cube like chunks. This type of rot is often called brown cubical rot. In both cases advanced decay can significantly reduce the strength of the tree resulting in fallen branches or trees that break or fall over. Trees often survive many years with heart rot. Internal rot often goes unnoticed for years because the tree can continue to grow with no symptoms of disease or decline in the canopy. Remember heartwood cells are not living cells and serve only to provide structural support for the tree. If basidiocarps are observed on the tree, it is likely that the tree has already been infected for several years. The decay caused by many heart rot fungi progresses very slowly, often at a rate of about 3 inches per year.

Assessing Damage in an Infected Tree

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Photo 3 (above): Fungi, indicating butt rot on maple. Michelle Grabowki.
A tree with signs of heart rot does not need to be immediately removed, but it should be further examined to determine how structurally sound it is. Special care should be taken with trees that are located near people or property that could be damaged by fallen branches or trees. In natural areas, trees with heart rot serve a valuable function in providing nesting sites for birds and animals and should be left undisturbed if possible. The stability of a heart rot infected tree will vary depending on the type of tree that is infected, the extent of the decay, the type of rot, and if other defects like cracks or cavities are present. The USDA Forest Service ( recommends that action be taken to stabilize or remove a tree if it has signs of a heart rotting fungi and one or both of the following:
  •  a crack, open wound or other defects are also present

  •  less than 1 inch of solid wood for every 6 inches of tree diameter.
The level of internal decay can be difficult to determine if an open cavity is not present. Gardeners concerned about the structural stability of a tree should contact the local city forester or a certified arborist ( These professionals can run a variety of tests to help assess the stability of the tree.

Preventing Heart Rot in Trees

Gardeners can prevent heart rot by protecting the trees most important natural defense - its bark.
  • Take care not to wound trees with lawn care equipment.

  • Protect trees from deer rubbing and rodent damage with fencing.

  • Use proper pruning cuts when pruning trees and never leave a branch stub (see:

  •  If branches are broken during a storm, cut damaged branches with a proper pruning cut below the damaged area. The tree will be able to heal a smooth pruning cut more quickly than a rough jagged rip.