Fungi and Trees

There are a number of fungal infections that attack and affect trees. There is also a number of fungi that are beneficial to trees. Lets look into some of the basics when it comes to fungi and trees.

The Good

The number of beneficial fungi that are out there living with and often in sync with our trees is extensive. There are fungi that live on the foliage that help, and fungi that decay old wood and generally are not harmful to the trees. But most importantly there are groups of fungi that live in the soil that have specific relationships with tree roots. These are known as Mycorrhizae. Mycorrhizae is not a specific fungus, it refers to the symbiotic relationship that a specific species of fungi has with the vascular tissue of a plant. So each different species of tree has its own species or groups of fungi that only associate a mycorrhizal relationship with that species of tree. Additionally, the species of fungi vary greatly among soil types and regions. So a specific species of fungi that helps, say a silver maple (Acer saachariunum) may be different across town or across the state.

Why is this important? A tree’s health can only be as good as the health of the soil it is growing in. Without healthy soils full of beneficial fungi and other bacteria, the tree will struggle to gain the necessary water and nutrients from the soil and grow the best root system it needs in order to thrive. In order to have, or get, healthy soils, follow these basic guidelines:

  • Reduce/prevent soil compaction.
  • Add decomposable wood mulch to areas around the base of trees.
  • Mulch grass clippings and leaves – The more organic matter the better.
  • Avoid a lot of synthetic chemicals or other chemicals that can kill beneficial fungi and bacteria.
Tar spot fungal infection on a Norway maple leaf. Looks bad, but really isn't harmful to the tree.

Tar spot fungal infection on a Norway maple leaf. Looks bad, but really isn’t harmful to the tree.

The Bad

Just as much as there is beneficial fungi, there is detrimental fungi. There is fungi that decays wood and some that even can decay live wood and structurally weaken trees. But there is also fungi that attack leaves, needles, twigs, fruit, and roots. Of the fungi that attack and harm trees or living wood, most (if not all but a special few) only can start harming trees if the trees are stressed and can’t fight of the infection on their own. I often explain it like our own immune system; The healthier we are, the less often we get sick. It is very similar with trees. As long as we try and focus on tree health, fungal infections to the leaves and other growing tissue is less likely. However, no matter what we do there are still some fungal infection that will occur depending on the weather. These are the fungi that often attack leaves, needles and/or fruit. Some can be harmful to the tree while some or mostly visually unappealing. I could go into a list, but then we would be here a while. So here are some basic guidelines when dealing with detrimental fungi:

  • Root and live wood fungi are often directly related to severe tree stress or damage.
  • ALL fungal foliar problems have to be treated proactively! Once you see the damage from the fungus it is too late for that season. Preventative sprays are about the only option for most foliar fungal infections.
  • Sterilize all pruning tools during the growing season if fungal infections of the twigs or wood is present (e.g. black knot, cytospora canker, etc.).

The Ugly

I suppose this comes after the Good and the Bad, but really the bad fungi is bad enough. The only thing I can think of for here is some of the wood decay fungi that can affect the structural integrity of trees. Its getting to that time of year (late summer early fall) when certain wood decay fungi will start reproducing. If you start seeing mushrooms or other fungal conks on the trunks of your trees or around the base, its is a sign of internal decay that should be checked out by a qualified arborist. Of course that is something that I can help with, but if your out of the Sioux Falls area, go to and search for a certified arborist in your area.


Not all fungi are bad, most are very good for trees and can make a huge difference between a tree that is thriving or just trying to survive.