Heartrot can be very painful.  This animation is much cooler in MS Internet ExplorerTom Volk's Fungus of the Month for February 2004

This month's fungus is Phellinus tremulae, one of the causes of heartrot,
in "honor" of Valentine's Day

Please click TomVolkFungi.net for the rest of my pages on fungi

Phellinus tremulae-- isn't it beautiful?

Maybe, like me, you've had enough of all the hearts, cards, candy, flowers and all the other commercialized aspects of the "Hallmark Holiday" that Valentine's Day has become. Our local stores were putting out the Valentine's Day cards even before Christmas this year!!! This month's fungus is the perfect antidote to all that.

To that end, I'm going to tell you about Phellinus tremulae, one of the many causes of heartrot in trees, in this case aspen trees. Phellinus tremulae is perhaps the most easily recognizable of the pathogens on trembling aspen (Populus tremulae, a.k.a. quaking aspen) and big-toothed aspen (Populus grandidentata), even though the aspens are among the most susceptible trees to decay. Heartrot can be particularly devastating if the tree is already under stress from some other environmental factor, such as drought or extremes in temperature. But what is heartrot?

Heartrot is the decay of live trees, mostly in the heartwood of trees. Heartwood can easily be distinguished from sapwood in a cross section of most trees. For example, below and to the right is a cross section of a juniper tree. The dark color in the center is the heartwood and the light color to the outside is sapwood. To remind you of your wood anatomy, all wood (secondary xylem) starts out as lighter-colored sapwood, where transport of water and dissolved minerals takes place. As the tree matures and grows in girth, more sapwood is added to the outside of the tree-- you can see this yearly growth as rings. As the tree grows, one major problem is how to dispose of waste products. Trees can't just dump their wastes out of their roots-- that's where they will be growing the next year. Unlike animals, trees can't move, so they can't dump their wastes and then go eat somewhere else. Instead the tree dumps its dark-colored waste products to the middle of the stem, where they build up in large quantities. Some trees also dump their waste products into their leaves, which fall off at the end of the growing season in temperate regions, but that's a whole different story.

Undecayed juniper log. Heartwood in the center is dark; sapwood on the outside is lightThe consequence of this is that heartwood becomes filled with wastes and no longer functions in transport-- all transport thus must take place in the sapwood. In many species of trees (such as chestnut and juniper) the heartwood becomes very resistant to fungal decay, and any wood decay takes place mainly in the sapwood. In other trees, such as aspen, the heartwood remains susceptible to decay, and heartrot is common. Very often heartrot can take place for dozens of years before a conk is even produced, so heartrot is much more common that as first appears. The early stage of rot is sometimes called incipient heartrot.

Heartrot is responsible for millions of dollars worth of damage every year to trees in the forest, in parks and in urban and suburban settings. Trees are weakened by heartrot, and are more susceptible to being blown over by the wind. If heartrot takes place in the first meter or so above the ground, this is sometimes called buttrot. (One can only hope that, for the tree's sake, "buttrot" is not as painful as it sounds...) Some kinds of fungi (such as Laetiporus cincinnatus) cause root rot, and these trees are also susceptible to being blown over. However such fallen trees can be seen with their roots tipped up above the ground rather than being broken off at the trunk.

Phellinus tremulae on Populus tremuloides (Quaking aspen) in Colorado Phellinus species are not known for their beauty, although they are very interesting. Most of them are shelf-like fungi, with a distinct top and bottom. The pores are usually borne on the bottom and are lined with basidia that produce basidiospores. Sometimes there are sterile cells, called setae, between the basidia. These setae are thick walled and pointed; they are thought to protect the basidia and basidiospores from insect damage. Most Phellinus species are perennial, with a new pore layer laid down on the underside of the fruiting body every year, usually up to 5 years, but sometimes for 10-20 years or more. Thus protection against insects is particularly important to these perennial fungi.

To the left is a typical fruiting of Phellinus tremulae, in this case on quaking aspen in the mountains of Colorado. The fruiting bodies of this species of Phellinus do not project very far from the tree and looks like a very sloping shelf rather than a shelf you would place objects on. Many of the other dozens of species of Phellinus form very distinct shelves, but others remain completely flat on the under surface of a fallen log. You can think of the sloping fruiting bodies of Phellinus tremulae as kind of an intermediate between these two extremes. You should be able to find at least four conks on this particular tree.

Hymenochaetaceae, A= Hymenochaete, B=Asterostroma, C=Stiptochaete D=Hydnochaete, E=Inonotus, F, G=Phellinus, H, I=ColtriciaPhellinus belongs to a family of fungi called the Hymenochaetaceae, which also includes poroid members Inonotus and Coltricia, toothed member Hydnochaete, and smooth members Hymenochaete, Asterostroma, and Stiptochaete. Although members of this family have different kinds of spore-bearing surfaces, they are held together as a group because all are xanthochroic (turn black in 3% KOH), have simple septa rather than clamps on their dark hyphae. All of them cause a white rot in wood. In addition many have setae or setal hyphae for protection. In the photos to the right you can see some of the genera that belong in the Hymenochaetaceae. Smooth spore-bearing surface A= Hymenochaete sp. from Alaska, B=Asterostroma ochroleuca (characterized by its star-shaped setae), C=Stiptochaete from Puerto Rico (characterized by its stipe or stem, coming towards you in the picture). Toothed spore-bearing surface D=Hydnochaete olivaceum, and Poroid spore-bearing surface E=Inonotus glomeratus from Indiana, F, G=Phellinus igniarius top and bottom, from Michigan, H=Coltricia perennis, I=Coltricia (=Cyclomyces) montagnei from Wisconsin. For most of these the genera are fairly easy to identify, but the species within the genera are very difficult because microscopically they can be very similar. Phellinus and Inonotus species can be particularly difficult to identify because there are usually few distinguishing microscopic characteristics. One very important criterion is the species of host tree, which is sometimes difficult to discern because of an advanced stage of rot. So although I'm not providing you with a key to the species, I've provided below an artificial key to the traditional genera of the Hymenochaetaceae.

The family Hymenochaetaceae is a prime example of how microscopic characteristics and DNA studies can be used to group species and genera of fungi together when they appear to be very dissimilar macroscopically, even to the point of having different ways of bearing their spores (hymenophore configuration). The microscopic characters (and DNA studies) make us go back and more closely examine relationships among these fungi and help us to understand how they might be related.

Artificial Key to the traditional genera of the Hymenochaetaceae

1.  Hymenophore smooth...................................................................2

      2. Fruiting body resupinate or effused-reflexed ........................Hymenochaete

      2. Fruiting body stipitate, tropical to subtropical....................Stiptochaete

1. Hymenophore poroid or toothed.........................................................3

     3.  Hymenophore toothed..................................................Hydnochaete

     3.  Hymenophore poroid..............................................................4

          4.  On the ground, mycorrhizal, centrally stipitate....................Coltricia

          4.  On wood, not stipitate.....................................................5

                5. Annual, softer (monomitic).....................................Inonotus

                5. Perennial, harder (dimitic to trimitic).......................Phellinus

For an explanation of some of the terms, please see this page on polypores, "Polypore primer: An introduction to the characters used to identify poroid wood decay fungi."

If you've had experience with trying to key out these species, these you know that it is sometimes very difficult to distinguish between Phellinus and Inonotus. In fact, according to modern studies the distinction is no longer valid, and the wood-inhabiting poroid members of this family have been placed into 14 (fourteen!!) genera. According to Tobias Wagner and Michael Fischer in Germany (Mycologia, 94(6), 2002, pp. 998-1016), the poroid genera include Phellinus s.s., Inonotus s.s., Inocutis, Fomitiporella, Aurificaria, Phylloporia, Fulvifomes, Mensularia, Pseudoinonotus, Fomitiporia, Porodaedalea, Onnia, Fuscoporia, and Inonotopsis. They base their conclusions on a combination of molecular and (to some extent) morphological studies. Until I can learn more, I will have to continue to use Phellinus and Inonotus. The mycorrhizal poroid genera Coltricia and its segregate genus Coltriciella continue to be accepted.

woodpecker holes made possible by wood decay fungiSo now you may be thinking, "Wood decay is bad. Killing trees is bad. All bad." However, that is certainly not true from an ecological standpoint. Wood decay fungi are important recyclers of nutrients, returning the carbon, nitrogen and other nutrients from the wood back into the soil and air, where they can be used by other organisms. We would be over our heads in undecayed wood if it were not for the wood decay fungi.

Rot of living trees is also important ecologically. The fungi that kill off weakened trees and cause them to fall down make "gaps" in the forest canopy that allow sunlight to come through to the forest floor, which in turn allows new trees to grow. Thus the pathogenic wood decay fungi play an important role in maintaining the health of the forest.

Even incipient decay provides some benefits. As decay takes place, the wood softens, allowing insects to bore into the tree. Woodpeckers can find this softened wood, seeking out the insects and grubs for food. Eventually the birds can peck out larger holes in the decaying wood, which provides them with nesting cavities. Eventually they abandon the cavities, which can then be used as nesting sites for many other kinds of animals. You can see some of these cavities to the left, along with a Phellinus fruiting body. And to think it all began with a wood decay fungus!

For more information on tree diseases and their impact on life in the forest, I recommend you sit Jim Worrall's fantastic Forest & Shade Tree Pathology-- forestpathology.org

heartrot resulting in a hollow logI hope you enjoyed learning something about Phellinus and other members of the Hymenochaetaceae. They're not the most beautiful fungi, but they serve important functions in forests as wood decay, especially heartrot. I also hope your own heart doesn't experience any rot this Valentine's Day...

If you have anything to add, or if you have corrections, comments, or recommendations for future FotM's (or maybe you'd like to be co-author of a FotM?), please write to me at volk.thom@uwlax.edu

This page and other pages are © Copyright 2004 by Thomas J. Volk, University of Wisconsin-La Crosse.

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