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Fungus on Trees

Fungus on Trees
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Fungi are a major part of every ecosystem and a key component to every life cycle.  Fungus is found in every healthy ecosystem from a forest to a garden, even within our own bodies!

While fungus itself isn’t a bad thing, when it’s found growing on a tree it indicates a major health issue for that tree. Some fungus like black knot fungus, Hypoxylon canker, or Dutch elm disease are aggressive fungi that target a tree and continually cause it damage until decline. Other fungi will take advantages of a trees weakness and cause that damage to turn to rot, ruining the trees integrity.

Fungi gain their nutrients by breaking down living tissues in the decay process. A fungal growth we recognize as a mushroom or conk is the reproductive organ of a fungus. At the point we can see a fungi’s fruiting body the fungus has caused a fair amount of decay inside the tree. Trees are specially adapted to resist decay by compartmentalization. They will lock off decaying parts of themselves with hardy nonliving tissues to slow the decaying process then fortify the weakened areas with new healthy tissues, this is how living trees can have hollows that animals use for homes or large cavities at their base. Some fungi and decay will make its way through compartmentalization so once a tree has decay within it’s usually a race between how fast the decay advances and how fast the tree can compartmentalize and outgrow the damage.

Fungal growths in a trees crown are often not going to kill a tree. The tree will compartmentalize the branch, letting it die off and eventually it will break off the tree. Some fungi target foliage instead of wood. Those fungi may cause a tree to decline if the infection is aggressive and defoliates the tree for an extended amount of time but do not affect the trees wood.

Fungus that has taken advantage of weakness in the trunk or roots are harder for the tree to fight. The tree needs both its roots and its trunk to maintain its form and to transfuse nutrients through out its body. It cannot completely compartmentalize either part, so this allows the fungus to continue its decay through the compartmentalized sections. Most trees can outgrow the decays progression in good conditions, what usually kills them is failure. Decayed wood either in the spongy and soft form of white rot or the brittle and delicate brown rot, is very weak. Rotted wood will fail under load causing the tree to fail at the point of weakness during a storm, snow load, or impact.

How to prevent it

Making sure your tree is healthy is the best prevention as that tree will be able to outgrow and survive damages, maintain a proper canopy density, move nutrients effectively, and keep a healthy cambium throughout the years. Make sure you tree gets the proper amount of water and sunlight for its species and that its soil composition is ideal for its needs.

Removing fungal opportunity is key in proper tree care. This means addressing damages to a tree so it can more effectively compartmentalize and keep its key structures rot free. Broken branches and stems should be removed as soon as possible. Deadwood within a canopy should be removed as well as that wood had already been compartmentalized by the tree and let die so its decay did not spread to the main tree components.

What to do

If fungi are found on the tree’s trunk it indicates a column of decay within. While that column may not kill the tree it is a weakness that could cause the tree to fail. If fungi are found on the tree base or exposed roots it indicates root rot, another weakness that could cause the tree to heave. If fungi are found on those parts of the tree, then its best to have an arborist inspect the tree and tell you how likely failure is. Then you can decide if the risk is acceptable or if the tree should be removed or pruned to lower its likelihood of failure.


Lichen is not fungi, nor is it moss. Lichen grows as part of the symbiotic relationship between some fungi and some algae. Lichen growths on your trees do not harm the tree in anyway and do not indicate anything about the tree’s health.