October 7, 2024

Wood-Decay Fungi, Part 2: Identifying Important Species

In part 1 of this article, “Wood-Decay Fungi, Part 1: Understanding Morphology,” which ran in the February 2024 issue of TCI Magazine, we 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 and types of decay – white rot, brown rot and soft rot – and we touched on location of decay.

Kretzschmaria deusta, or brittle cinder, forms a crust ranging in color from gray to white when young and changing to black and brittle with age.

Kretzschmaria deusta, or brittle cinder, forms a crust ranging in color from gray to white when young and changing to black and brittle with age. Older fruiting bodies look similar to charred wood. All photos courtesy of the author.

In this article, 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.

I have ordered the following fungi for identification from what I consider the most dangerous to the least, relatively speaking. And that’s taking into account the type of rot they create, as well as where they occur on the tree and how easy it is to assess the amount of decay they cause.

Please note that this list is not exhaustive, nor does it mean that the fungi at the “bottom” of the list can be ignored. These are all wood-decay fungi, and each situation in which they are found should be examined and met with caution on an individual basis.

Brittle cinder fungus, Kretzschmaria deusta (old names: Ustulina vulgaris, Hypoxylon deustum)

Let’s start off with brittle cinder fungus, also known as Kretzschmaria deusta, what I consider to be one of the most dangerous fungi on this list. Obviously, Latin names are important, but common names often tell you the key characteristics of what you’re looking at. Thus, brittle cinder fungus makes the wood look like cinders or burnt wood, and it is a soft rot, so it creates a brittle tree.

Its host is mostly hardwoods; I have found it on honeylocusts, oaks, shagbark hickory, beech and maples. It’s very inconspicuous, which is one of the reasons I think it’s one of the more concerning ones. This fruiting body is a two-dimensional, crust-like structure that lays flat on the bark.

Earlier in the season (April-June) it has a white border and light-greyish coloring, is velvety to the touch and sometimes will weep amber liquid. As it ages, or as the season progresses (July-August), it will go to a black, warty-looking texture that mimics the look of burnt bark. It can trick people into thinking it’s just wet wood from slime flux or wounds if not inspected closely enough. For those cases, try peeling it off. If you peel off brittle cinder fungus, there will be cambium/necrotic bark underneath. With slime flux/wet wood, there will be nothing to peel off on top of the bark.

Another concerning thing about this fungus is its location. In nearly all the instances in which I have seen it, it is right on buttress, right where the trunk meets the soil.

This is a serious disease, and the presence of this fungus can be an indication of risk. A host tree should be considered for removal (recommended with crane for safety reasons). Aboveground symptoms are often not present, and there typically is not a hollow sound associated with this fungus when the tree is struck with a sounding hammer.

Mostly the morphological identification of the fungus is the only way you would know it was there. And because it lays flat to the bark, it can hide under snow and groundcover. Always do a thorough visual check (including moving any obstructions) before work on or under a tree. This is really good practice for any time one is working around/in trees, not just when brittle cinder fungus is present.

Fruiting body (sign) of chicken of the woods.

Fruiting body (sign) of chicken of the woods.

Chicken of the woods, or sulfur shelf (Laetiporus sulphureus)

I’ve listed chicken of the woods after Kretzschmaria deusta because it’s a little more obvious when it’s present. This fungus creates a brown rot, which is like soft rot, that will result in a brittle tree.

Chicken of the woods is easier to identify because of the form it takes. It is a shelf-like fungus that grows up the tree, and it is usually, though not always, bright orange with a yellow margin on the cap and a yellow pore side. Note that in some instances, the cap can present in more of a beige or salmon color. This is technically Laetiporus cincinnatus, but it still is often called the common name, chicken of the woods. Except for the color, all other identifiers are the same.

On this fungus, the pores are very tiny, so it looks like they’re not there. This gives the underside a velvety look. This is very important, because one of its main lookalikes, Hen of the woods (Grifola frondosa), has much bigger pores in comparison. The texture of chicken of the woods is very fleshy, almost the consistency of tofu. As to where it grows, I’ve noticed it tends to associate with cavities and cracks, so it’s not necessarily in the same place on the tree. I’ve seen it up in the canopy; I’ve also seen it in the trunk, as well as the buttress area.

Chicken of the woods attacks hardwoods, especially oaks. Aboveground symptoms are often not present. Annual conks usually appear in late summer through late fall.

Honey mushroom root-rot sign.

Honey mushroom root-rot sign. Fruiting bodies, or the colloquial term mushrooms, are common signs of wood-decay fungi.

Armillaria root rot or other common names: Shoestring root rot, honey mushrooms (Armillaria sp.)

Armillaria root rot is third on this list only because it’s a white rot in comparison to the brown and soft rots I’ve already discussed. That makes it less concerning, relatively speaking, as far as the tree structure is concerned, but it’s high up on this list because it’s a root rot. The extent of root rots is fairly difficult to determine when doing a tree-risk assessment unless you are planning on excavating the root system.

The other reason it’s higher on the list is because the identification of the mushroom Armillaria forms can be quite difficult. Mycologists have a joke of identifying myriad mushrooms under the umbrella term of “little brown mushroom,” because there are so many fruiting bodies that look just like little brown mushrooms and are difficult to distinguish without further microscopic or genetic analysis. Armillaria can fall under that category.

If conducive conditions are present, Armillaria grows in clusters or a bouquet of fruiting bodies that have a tannish round cap and lighter to creamy tan gills. Yes, gills! So far, we’ve only talked about pores; but Armillaria has gills, and that separates it from the others on this list.

Armillaria gills can alternate from one gill that goes all the way from the stock to the cap, to the next gill that will be a false or half gill. That’s the best way to try to identify Armillaria, but for the most part, a laboratory or an expert is needed to fully confirm.

The bouquet can be growing directly on the buttress system or away from the trunk, seemingly not associated with the tree. But it is, of course, still associated with the tree – remember, Armillaria is a root rotter, so it can grow directly off the roots, too. Sadly, the mushroom doesn’t always make an appearance, but if it does, it’s in August/September.

Remember, though the fruiting body isn’t there, the vegetative body is! Thus, another sign Armillaria produces is mycelia fans or mycelial mats. Mycelia are just a bunch of hyphae accumulated together, and they form a nice little white fan underneath the bark. They’re not obvious, so you do have to go looking for them, and in some cases you must chip the bark off to find the mat.

One other sign is rhizomorphs. These look like shoestrings, or black roots, literally growing up underneath the trunk of the tree. These are why this fungus is sometimes referred to as shoestring root rot. Once you start seeing rhizomorphs, typically the infection is a bit far along and the bark will be sloughing off – no chipping usually needed.

Armillaria has a wide host range and can spread around through root grafts when present. Trees infected with this fungus have obvious symptoms of decline.

Weeping conk

Weeping conk: When “young,” it could look nice and fresh, with a creamy-colored cap conspicuously weeping amber liquid.

Weeping conk or oak bracket (Inonotus dryaedeus syn: Pseudoinonotus dryadeus)

Here we have another white rot (flexy tree), and it’s a root rotter just like Armillaria, which means it can be really difficult to detect the extent of decay. To add to the pile, it can be difficult to identify, though not as difficult as Armillaria. Ease of morphological identification depends on when in a weeping conk’s life cycle you find it.

Weeping conk is an annual, which means it completes its life cycle all in one year. When “young,” it could look nice and fresh, with a creamy-colored cap conspicuously weeping amber liquid (as the common name would suggest) and a creamy-colored pore side. But if you find it when it’s “old,” this fruiting body looks crustier and sort of blackened/mahogany in color, with holes but no weeping liquid.

Like Armillaria, it can be present in the grass because it’s also a root rot. Typically, when I find it in this location, it will look more like an amorphous blob with no discernable shape. It also can grow directly on the buttress system of the tree and the form is more shelf-like, in what I call a pancake-with-maple-syrup form. No matter where it is, always remember that old conks turn black and can remain for several years.

Trees infected with this fungus have aboveground symptoms of decline. Keep in mind, there is a period of time when the tree will have symptoms, but the sign is not present yet. So always thoroughly inspect the tree and roots if possible. It is found mostly on hardwoods and seen a lot on oaks.

The fruiting bodies of G. applanatum, or artist’s conk,

The fruiting bodies of G. applanatum, or artist’s conk, are perennial, and may persist for multiple years, increasing in size and forming new layers of pores as they grow.

Ganoderma root and butt rots: Ganoderma applanatum and Ganoderma sessile

Next up, we have artist’s conk, or Ganoderma applanatum. Artist’s conk is a moderate to aggressive white rotter, but it’s lower on the list because it’s easier to identify. Also, it’s not a root rotter, so we can more easily detect the extent of decay with a sounding hammer, resistance drill or sonic tomography.

This is a white rotter, so it creates a flexible tree. You do get some good canopy decline from trees infected with this fungus as well. It can be found anywhere from ground level to 6 feet up on the trunk. I will often see this on oaks and honeylocusts, but it can infect many hardwood trees.

This fungus is also distinctive from a morphological standpoint. It’s a woody, shelf-like fungus with a creamy pore underside and a tan to light-brown top side with ridges. It is a perennial, which means it persists over winter and continues to grow multiple seasons. Interestingly, you can count the ridges or spore layers it puts on every year like rings on a tree. You’ll have to rip the conk off the tree to look at these internal layers – try not to do this in rainy/humid weather (more on that when we discuss management).

Why is the common name artist’s conk? Because you can easily bruise the underside and leave a mark, making it a very good canvas. Gently run your finger over it and it will leave a line that lasts through the drying process (so choose your inscription wisely). Some people use this as an identifying characteristic, but plenty of shelf fungi can bruise. I prefer using the layers method to identify, as well as the color and shape.

Ganoderma sessile

Ganoderma sessile can be found anywhere from ground level to 6 feet up on the trunk.

We definitely want to know if we have Ganoderma applanatum, or artist’s conk, versus its potential lookalike, Ganoderma sessile. Ganoderma sessile also is known by its common name, varnish conk. Varnish conk is not as aggressive and is lower on the list. It causes a white rot and is a buttress and root rot. There’s typically not a lot of decay associated with varnish conk in comparison to artist’s conk, but it’s certainly something to keep an eye on.

I generally will see varnish conk on honeylocusts, but it also could be on a lot of other hardwoods, such as oaks.

Varnish conk is an annual fungus, so like weeping conk and brittle cinder fungus, its morphology will change throughout the year. When it’s young, it starts off like a white marshmallow where you see mostly the pore side, or underside, that’s creamy. You could bruise this side as well, though it’s not as easily bruised or permanent as on an artist’s conk. As it ages throughout the year, it does unfurl and form a nice woody shelf.

And this is where the most distinctive features for this fungal identification appear. Hopefully by now you have noticed the usefulness of the common names. And the pattern continues with varnish conk. The most identifiable feature is that the texture of the cap is shiny or varnished. At this stage, varnish conk is a mahogany-red, kidney-bean-shaped conk with a shiny textured cap. But be warned, both color and shine fade with age. That is why it is important to note that Ganoderma sessile doesn’t form layers. That’s the main way to differentiate between artist’s conk and varnish conk.

Management

Can we manage these fungi? The answer is sort of, but not in the traditional way people may understand management. We cannot stop wood-decay fungi once they infect, and that’s probably one of the most difficult things to talk to a client about. Once a wood-
decay fungi infects a tree, its decline and death are inevitable. The only thing we can do is slow the decline and delay the inevitable.

So how can we slow things down? Wood-decay fungi are stress-related organisms, which require a host to be weakened or wounded to infect. Thus, there are two options: prevent any wounding from occurring or prevent/mitigate stress on the host.

In the latter case, what we’re doing is trying to create a situation in which the host, the tree itself, can defend itself and compartmentalize the infection. And that is going to greatly depend on the condition of the host when you stepped on site. Client-expectation management is incredibly important if you attempt to manage a tree infected with wood decay.

Controls

Slowing down the infection can be attempted using cultural methods. More specifically, look at water management and soil care to mitigate issues like compaction, poor drainage, low organic matter, low biodiversity, etc. The most important thing when considering soil care for management of wood decay is the time it may take for these amendments to have effect. Generally, using soil care as a preventative measure rather than reactive should be considered.

While phosphonate products (such as Reliant, Agri-fos, etc.) have Armillaria root rot on their labels, it’s very important to remember that phosphonates are host-defense activators and not conventional fungicides. With phosphonates, you are affecting the host directly in an attempt to increase detection and compartmentalization against the fungus. This makes phosphonates management of wood decay more similar to cultural management than a conventional fungicide. It is important to keep that in mind so the client can have their expectations set appropriately when using a product like this.

Don’t destroy the evidence!

Destroying the reproductive fruiting body only stops reproductive dispersal and not the vegetative structure of the fungus. Remember, it is the vegetative structure that is absorbing nutrients and actually causing the rot. Removing the fruiting body is just stopping the spores from spreading, that is, unless you’re ripping it off during rain or high humidity, in which case you may be spreading the spores instead. Destroying the fungal fruiting bodies is fine to stop the spread reproductively, but it’s not going to stop the hyphae from moving into the tree and decaying it.

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, click here.

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