Trees are living organisms that are capable of being injured. Animals, parasites and weather all put their resistance to the test. Trees are also susceptible to disease. Some of these diseases are of bacterial origin, while others are caused by insects or fungi. However, the presence of fungi on a tree doesn’t necessarily signify its imminent demise. Arboriculturists and botanists have demonstrated that some types of fungi can inhabit a tree without causing any significant damage, while others can actually be beneficial.

The good and the not-so-good

Fungus that’s visible at the base of a tree or on the bark is more or less analogous to mold, which implies a certain amount of decay. However, there are numerous species, families and genera of fungi. And their impact on trees and other plants can vary greatly. For example, microscopic mycorrhizae form a symbiotic relationship with a tree by colonizing its roots, helping it absorb nutrients more efficiently. Another beneficial type of fungi, the ascomycetes, form a large division of sac fungi, some of which create a humus that’s favorable to the tree and also highly prized (e.g. oak tree truffles). In Quebec, edible polypores choose to take up residence primarily on the trunks of maple trees and certain strains of hemlock.

Looking at the relationship between fungi and trees from a standpoint of arboriculture, rather than mycology, it’s important to note that the presence of fungi can often lead to parasitic attacks, which can be surmounted or at least controlled through appropriate pruning. Trimming a tree—whether to remove dead, diseased or damaged branches or simply to thin it out—makes the tree stronger and more resistant.

Types of fungi

In the woods, a mushroom growing on a tree might seem normal and even lend some special enchantment to the setting. However, you should be aware that several types of fungi develop on trees that are weakened, wounded or in the process of necrosis. The effects of these fungi, which may progress very slowly, are visible on the bark and leaves. Appearing as cankers, blemishes and defoliation, these are not such enchanting sights. Let’s take a look at three different ways in which fungi develop.


Many think that fungus is synonymous with rot and decay. However, it can play an important role in the growth of certain trees. In fact, many types of fungi that grow on the forest floor have a symbiotic relationship with the trees. How? Through an exchange of nutrients. This symbiosis is called mycorrhiza. Mycorrhiza is an association between a tree’s root system and the vegetative organs of the fungus. The fungus provides the tree with water and minerals it can’t extract from the soil on its own, while the tree provides the fungus with nutrients it can’t synthesize.


We’ve all noticed fungus on wood that is dead or rotten and on trees that are in poor condition. This is known as saprophytism. In a nutshell, the fungus plays a role in the decomposition process by digesting the organic material, which is ultimately returned to the soil. As we all learned in science class, “Nothing is lost, nothing is created, everything is transformed.”


Parasitic fungi attack or attach themselves to a host (a tree, a plant, etc.) that may be in good health or may be already diseased. A parasitic fungus will often graft itself onto a tree for many years without affecting its growth, while other times, it may speed the death of a tree that’s already sick. However, you should be aware that parasitic fungi are very often microscopic.

If you notice fungi on your tree and the tree is showing signs of disease, perhaps it’s time to think about contacting a professional. So don’t hesitate to ask our team to come and evaluate your trees if you have any concerns about their state of health.

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