Wood-Decay Fungi Part 1: Understanding Morphology

In this article, we are going to talk about fungal anatomy as well as how and where fungi (fun’ ji) attack trees, and how that in turn affects the trees and us.

a tree that’s infected by a root-decay fungi
When we’re looking at a tree that’s infected by a root-decay fungi, most of the time we will see dieback and canopy discoloration up in the very top and moving downward throughout the tree. All photos courtesy of the author.

Then, in Part 2, we’re going to get into some species-specific identification.

A quick note about the species I’ve selected. They are mostly northern and hardwood issues. Just be aware that if you are in the southern or western regions of the U.S., or if you work with palms, unfortunately there is other wood decay out in the world that I can’t cover.

Fungal anatomy

Just as with tree identification, understanding fungal anatomy is very important for fungi identification. And when I say fungi, I really mean fruiting bodies. The colloquial term, of course, is mushrooms. But technically, not all fruiting bodies are mushrooms. It might be semantics, but just know that when I say fungi identification, I’m talking about fruiting bodies or mushrooms from here on out.

When we are trying to identify a mushroom, we’re going to consider all of the mushroom parts, just as with tree ID when we take into account the bark, tree form, branches, leaf attachment and leaves themselves – these are all identifiable characteristics. The same things exist for fungi.

I want you to first take note of the color of all the parts, so the color of the cap or top, the part that faces the sky; the color of the underside, or the spore side if you prefer; and the color of the stock or stem, if it has one.

We also want to observe the texture of all those parts. Some fungi or fruiting bodies can have scaly tops, some can be weeping or bleeding and some of the stems also can have similar characteristics. Thus, color and texture of all parts are very important.

mushroom parts
Just as with tree ID when we take into account the bark, tree form, branches, leaf attachment and leaves themselves, we need to consider all of the mushroom parts for fungi ID.

Shape and form

The shape itself mostly applies to the actual cap. Is it round, is it more vase or funnel shaped or is it triangular or fan-shaped? Does it have curled-up edges? Or are they downturned?

Does it have a stalk or a stem? In some cases, the identification between species morphologically is based purely on whether it has a stem or not.

The way we learned elms tend to form a vase shape, form is also an identifiable characteristic with fungi. Some fungi form what’s called shelves, and that’s just like a shelf on a wall that we could put a trinket on.

Then there’s bouquet fungi, which are a bit more complicated or at least harder to identify. These fungi form from a central point where you can sort of gather them all up in one hand and hand them to someone like a bouquet of flowers.

Obvious sign of fungi is the fruiting body
The most obvious sign of fungi is the fruiting body, which in this case is a capped mushroom. This structure is the reproductive body of the fungus that disperses progeny.

Parasite or saprophyte

There are plenty of fungi that are not parasitic. Parasitic describes something that infects or kills live tissue. What we’re concerned about are the wood-decay fungi that kill live tissue. There are other fungi, like mycorrhizae, that are helpful to trees and associate with the roots and soil, and we want them to be there.

There also are saprophytes. Saprophytes decay already-dead matter, and are nature’s recyclers. We’re not too concerned when we see them near a tree, but when we notice they’re on the root system, the trunk or the branches, it is important to remember that, while they are not causing active decay, they do live on dead matter. Saprophytes can be a “red flag” for decay or dead tissue in trunks or limbs.

Time of year

Time of year is another important observation. Some fungi will only come up during, say, August to September. Some fungi can produce fruiting bodies during January. Noting the time of year can help eliminate fungi that “bloom” in different seasons.

Also related to time of year is questioning if the fungi are perennial or annual. As with a plant, perennial means it comes back after one year or completes its life cycle in multiple years or seasons. Annual means its life cycle comes and goes all in one year or season.

As many of these characteristics should be noted as possible, since this will greatly help with identification.

white mycelia
Another sign of fungi – the white mycelia (in center of photo). Mycelia form the vegetative body of the fungus that explores substrates and absorbs nutrients.

Diagnosing decay (symptoms and signs)

Let’s talk about what the tree looks like when it’s being attacked by a wood-decay fungus. Remember, we are focusing on the parasites, or the fungi that can flip into parasites, because some fungi can be saprophytic and then switch to parasitic when a tree gets stressed. That’s why they’re so hard to manage. Since we’re focusing on the parasitic side of things, there will be an infection/cell death which ultimately means there will be symptoms!

Symptoms of wood decay

Remember, a symptom is a plant’s reaction to something disruptive.

When we’re looking at a tree that’s infected by a root-decay fungi, most of the time we will see decline. This is due to the fungi degrading the vascular system, which means the tree cannot conduct proper nutrient and water transport. As a result, you tend to see a lot of dieback and canopy discoloration up in the very top, or even in the tips of the branches and then moving its way backward and downward throughout the tree.

The same symptoms would occur if we’re dealing with a vascular wilt, a girdling root or construction damage; they all express the same way, because they’re doing the same thing to a tree – cutting off the water and nutrient transport.
As the name would suggest, another symptom is rot. This is wood decay, so there should be decayed wood! I say it all the time, scientists are not creative people. We tend to name things after what they look like, and that’ll become even clearer as we get into fungal identification.

Pseudoinonotus dryadaeus, or weeping conk
Pseudoinonotus dryadaeus, or weeping conk, causes white rot and decay of the trunks.

Signs of wood decay

Recall that the sign is the physical presence of the disruptive agent. In this case, it is the fungi itself, present as the fruiting body or the hyphae/mycelia.

Let’s quickly go over fruiting body versus hyphae/mycelia. Most fungi have two bodies: a vegetative body that devours nutrients and a reproductive body that disperses progeny. Generally, the vegetative body is the hyphae or mycelium. The reproductive body is the fruiting bodies or mushrooms (technically, a fruiting body is made up of hyphae).

It might be easier to think of the fruiting body as the “flower” (mushroom), which is producing “pollen” (spores), whereas the hyphae is the vegetative growing structure that absorbs nutrients, like the root system of the flower/mushroom.

Or, if you watched the HBO show, “The Last of Us,” the hyphae/mycelia are those very creepy tendrils growing beneath the skin (or out of the mouths) of the infected, while the fan-like crust covering the clickers’ eyes is the fruiting body. This show desperately needed a pathologist/mycologist to consult on the design of all the fruiting bodies that make an appearance throughout, as, frankly, the morphology is all over the place. I mean, how many different species of human-devouring Cordyceps are we dealing with here? But I digress!

For identification purposes of our wood-decay fungi, it is important to note that sometimes fruiting bodies aren’t there, which can make things tricky. Take, for example, Armillaria root rot. Armillaria root-rot fruiting bodies are very ephemeral; they only come up during the absolute perfect conditions, and often those don’t occur. However, Armillaria root rot also lives in its other form, hyphae. Hyphae form the root system of the fungi, and this is part of the fungi degrading the tissues. The fruiting body is just a sexual structure that’s dispersing spores.

Just because you don’t see it …

Expanding upon this, just because the mushrooms are not there does not mean the tree is not infected, because hyphae are present, and they are much less obvious. Wood-decay fungi can live and travel throughout the soil, just like other fungi, including mycorrhizae. But until you see those fruiting bodies or symptoms of the actual rot itself, you might not know it’s there for a long time. That’s important to take into account whenever you’re doing a risk assessment.

Types of decay

We’ve discussed the basics of fungi – they’ve got their growing body and their reproductive body – but how are they actually attacking the tree? In what way are they decaying the wood?

First off, there are two general components to the structure of wood, lignin and cellulose. Lignin supplies the rigidity of the wood; it’s what allows it to stay upright. Cellulose supplies the flexibility of the wood, allowing it to have a bit of give. Overall, structurally sound wood has a little bit of rigidity and a little bit of flexibility. What wood decay does is target one or more of these components.

Brown rot and white rot
Brown rot and white rot, aptly named for the color the wood appears as it decays.

White rot

Based upon which component(s) is targeted, we can get three types of decay. The first one is white rot, called such because – scientist creativity warning! – the wood looks white. This is the most common type of decay in hardwoods. White rots degrade the lignin and erode cellulose. But the main point is, it degrades the lignin first. Since it’s targeted lignin, it means we’re only left with cellulose, which means we’re dealing with a spongy, super-flexible tree. If you were to poke white-rotted wood, it would feel like a sponge.

Brown rot

Now contrast that with brown rot, which, as the name would suggest, makes the wood brown. This is more frequently found on conifers, but can be found on some hardwoods as well.

What brown rot mainly does is decay the cellulose. We’ve already established that cellulose causes a spongy flexibility to the tree. So when it is degraded, what we’re left with is just pure lignin. A tree that has brown rot is basically a tree that has all rigidity and no flexibility, which means a brittle tree. Typically, between these two types of rots, the brown rot is considered more dangerous because of the way the tree will fail. It will snap instead of bend. But both rots should be of concern when considering risk.

Soft rot

Lastly, we have soft rots, which are not visibly different from other rots (sorry), so it’s hard to tell what kind of rot you have just by looking at the wood. Soft rots are more similar to brown rot in the way they attack a tree. It’s good to know there is a difference, but soft rot and brown rot both primarily attack the cellulose, which means they both create rigid, brittle trees.

Location of decay

Roots, buttress or up in the canopy? It is important to know where these fungi are working, for reasons related to risk. For example, there are potentially different concerns and mitigation with the whole tree failing versus a branch coming down. Furthermore, if you have a root rotter, it’s a lot harder to know how many of those roots are rotten without excavation, versus rot in the buttress or the canopy where you can test with tools.


This is a good spot to end Part 1 of this article. We’ve covered fungal anatomy and the various identifying characteristics of fungi. These include shape and form, parasites versus saprophytes, time of year, the difference between symptoms and signs, types of decay – white rot, brown rot and soft rot – and touched on location of decay.

In “Wood-Decay Fungi, Part 2: Identifying Important Species,” in an upcoming issue of TCI Magazine, we’ll look at which characteristics help identify some of the fungi that can create hazardous trees for arborists. We’ll also look at fungal management and managing client expectations for trees with wood-decay rot.

Chelsi Abbott is scientific advisor/education specialist at The Davey Institute, a division of The Davey Tree Expert Company, based in Chicago, Illinois. Davey Tree is a 50-year TCIA member company based in Kent, Ohio. Abbott received her master’s degree in plant pathology from Purdue University. She is an ISA Certified Arborist and Tree Risk Assessment Qualified, an Association of Nature and Forest certified forest therapy guide and an adjunct faculty member with the College of DuPage in Glen Ellyn, Illinois.

This article is based on her presentation at TCI EXPO ’23 in St. Louis, Missouri. To listen to an audio recording created for that presentation, go to TCI Magazine online at tcimag.tcia.org. Under the Resources tab, click Video. For February Digital Edition click here.

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