Trees are living things that are susceptible to injury. Animals, pests, and weather test their resistance. Trees are also subject to disease. Some originate from bacteria and insects, others are fungal.
But the presence of mushroom on the tree does not necessarily mean its accelerated decline. Arboriculture and botany have shown that some types of mushrooms are beneficial to them and others can “inhabit” them without too much damage.
Which mushrooms are good for trees? (And not good)
A mushroom on a tree, visible at its base or on the bark, basically means mold, and therefore a level of decay. However, mushrooms are divided into a multitude of species, families, genera, etc. Their impacts are varied.
Microscopic, mycorrhizae colonize the roots in symbiosis with the tree, which thus better absorbs nutrients. Another example, ascomycetes form a large division of mushroom, some of which create humus that is favorable to the tree and are highly sought after (e.g., the oak truffle). In Quebec, edible polypores make their home on the trunks of maple trees and on hemlock stumps.
It is from the point of view of arboriculture and not of mycology that mushroom -a somewhat obsolete term, because it is very imprecise- are discussed here in their relationship with trees. Their presence often leads to parasitic diseases that can be overcome, if not controlled, by appropriate pruning. Pruning trees (sanitation or thinning pruning) makes them stronger and more resistant.
The different types of mushroom
In the woods, mushrooms on a tree may seem normal and make the place more enchanting. However, it is important to know that many types of mushrooms develop on weakened, injured or necrotic trees. Their slow-developing effects are then visible on the bark and leaves. And it’s not so pretty: cankers, spots, defoliation, etc. Let’s take a look at three mushroom development techniques.
It is often said that mushrooms are synonymous with rot and decay. However, they play a major role in the growth of certain trees. In fact, many mushrooms that grow on the forest floor have a symbiotic relationship with trees. How does this work? Through an exchange of nutrients. This is called mycorrhiza. The mycorrhiza is the association between the extremities of the roots of a tree and the vegetative apparatus of the mushroom. Thus, the mushrooms provides the tree with water and minerals that the latter cannot obtain from the soil and the tree provides the mushrooms with nutrients that it cannot synthesize.
We have all noticed the presence of mushrooms on dead wood, rotting wood or trees in poor condition. In these situations, we speak of saprophytism. That is to say that mushrooms play a role of decomposition. In short, they digest the organic matter that will return to the earth. As we all learned in our science classes, “Nothing is lost, nothing is created, everything is transformed”.
Parasitic mushrooms attack or attach themselves to a host (tree, plant, etc.) that is healthy or already sick. Sometimes, a parasitic mushrooms will graft itself to a tree for many years without altering its growth, while at other times, it may accelerate the death of an already sick tree. However, it is important to know that parasitic mushroom are often microscopic.
Should we react to a mushroom attack on a tree?
Not necessarily. The heartwood of a tree is always dead anyway. Only the bark and a thin layer under the bark (sapwood) are alive. The heartwood is still useful, as it helps the tree to better resist the wind, but its reduction or disappearance does not always have unfortunate after-effects: we often see mushrooms-infested and even completely hollow trees that are very strong. Often, they continue to live for 20, 40 or even 60 years after the first polypores are found.
To verify if your tree is still structurally sound, you should seek the advice of an experienced arborist. If it is considered unsound and in danger of falling over, it should be removed. Otherwise, let it continue its life.