Why Do Mushrooms Grow On A Tree?

A call I get occasionally during the year is: “Should I be concerned about mushrooms growing on my tree?”

This is nature being nature, and it may, or may not, be a cause for alarm. This is a little technical, but let me explain in detail.

Why Do Fungi Feed on Trees?

It all comes down to sugar. Like all living organisms, fungi need carbon, hydrogen, and oxygen to survive and build their structures. When we eat carbohydrates, our bodies break down the sugar molecules, repurposing the carbon, hydrogen, and oxygen into fuel. Even your car operates on hydrocarbons, a mix of carbon, hydrogen, and oxygen. Similarly, trees are made of these elements, which form complex molecules like cellulose and lignin. The tree’s skeleton consists of cellulose, a long chain of sugars, and lignin, which holds the cellulose together, providing strength.

How Do Fungi Feed on Trees?

Fungi use enzymes to break down these complex molecules, rearranging them to create their own structures called mycelium. These structures are long, interconnected threads that spread throughout the tree. When basidiomycete fungi reach maturity, they produce fruiting bodies (mushrooms) that contain spores. When temperature and humidity are favorable, the spores are released into the air. The mushroom itself is just a small part of the larger fungal organism. Think of it like an apple on a tree—only a tiny part of the entire organism, with the tree representing the mycelium.

Can Trees Defend Themselves Against Fungi?

Yes, trees can defend themselves if they are healthy and have the resources to do so. The first line of defense is the bark, which keeps fungi from entering. However, if the bark is damaged by storms, equipment, or improper climbing practices, fungi can find an entry point. Similarly, damage from storms, improper pruning, or root damage during construction can invite fungi. If the tree is healthy, it can isolate the infection through a process called compartmentalization, preventing the fungus from spreading throughout the tree.

Mushrooms Growing on Your Tree

If you notice mushrooms growing on your tree or around its roots, they likely belong to a group of fungi called basidiomycetes. These fungi are often referred to as "club fungi" because their spores are produced on club-shaped structures called basidia. Within this group, there are saprophytes, parasites, saprophytic parasites, and beneficial fungi. Recognizing the type of fungi is important for assessing the health and safety of your tree, which is why an ISA Certified Arborist should be consulted.

Saprophytes

Saprophytic fungi are beneficial because they break down dead plant material such as fallen leaves and rotting wood. These fungi are essential for nutrient cycling in the soil and the global carbon cycle. While saprophytes may enter a tree through wounds and decay the heartwood (the dead wood inside the tree), they don't harm the living parts. It's common for a tree to be hollow but still structurally sound, similar to a building that is hollow inside but still safe to occupy. A certified arborist can assess whether the tree is still safe.

Parasitic Fungi

Parasitic fungi feed on the living parts of a tree, especially the cambium, the layer just beneath the bark that moves water and nutrients throughout the tree. This area is rich in nutrients, making it an attractive target for certain types of fungi. Feeding on the cambium can lead to severe structural damage, decline, and even death. It’s common for parasitic fungi to go unnoticed until the damage is extensive, often without visible symptoms like dieback or mushrooms. A tree with this type of fungi can fail structurally without warning.

Saprophytic Parasites

Some fungi primarily feed on dead plant material but can switch to parasitic behavior under certain conditions. A common and dangerous example is Ganoderma sessile, which feeds on both living cellulose and lignin, as well as dead heartwood. These fungi often announce their presence with large polypore mushrooms growing at the base of the tree, indicating potential structural risks.

Beneficial Fungi

Mycorrhizal fungi form a symbiotic relationship with plant roots. They attach to the roots or form sheaths around them, extending into the soil further than the plant roots to collect nutrients like phosphorus. In exchange, the tree provides the fungi with carbohydrates. This mutually beneficial relationship helps both the tree and the fungi thrive.

Occasionally, mushrooms from these fungi may appear near the tree’s roots, which is generally a good sign of tree health. While not all mycorrhizal fungi produce mushrooms, their presence indicates a healthy ecosystem. These fungi are also often used in conjunction with deep-root fertilization for urban trees, as city soils tend to have fewer mycorrhizal fungi.

What to Do if You See Fungi in Your Tree

If you see fungal growths on your tree, consult with an ISA Certified Arborist. These experts are trained in identifying different types of fungi, their potential risks, and how they might affect your tree’s health and structural integrity. If the fungus cannot be identified, it can be sent to a lab for positive identification. This identification can let you know how harmful the fungus can be to your tree or if it is a mycorrhizal fungi.

Some arborists use tools like the Resistograph decay detection tool to assess the amount of decay in a tree. Just because fungi are present doesn’t mean your tree needs to be removed. Many treatment options are available to improve the tree’s health, mitigate risks, and sometimes save the tree—often at a lower cost than removal.

Trees are valuable assets to our environment and communities. Keeping them healthy enhances the beauty and value of your property. With proper care, many trees can thrive despite the presence of fungi.

If you see signs of fungal growth, reach out to one of our ISA Certified Arborists to ensure your tree remains a safe and beneficial part of your landscape. You can reach us by sending us a message through our website or give us a call at 800-724-6680.