Introduction to Crust Fungi

Overview

Besides the old rice burnt onto the bottom of your pot, crusts — or corticioid fungi — are of great morphological and evolutionary diversity, as well as ecological importance. You can find them almost anywhere where there is wood, growing on the underside of a stick or log, on the bark of living trees, or even from the ground. They make their living as saprotrophs, eating dead organic matter, or biotrophs, gaining nutrition from a living host. In the latter case, they can be mutualistic, resulting in reciprocal growth and survival benefits for the plant host and the fungus, or pathogenic, thriving at the expense of the plant. While mushroom hunters often scoff at crusts, fungal ecologists increasingly appreciate their importance for forest health and ecosystem functioning.

I'll be the first to admit that these aren't the flashiest of organisms. They are usually small and indiscrete, some nothing more than a sheen or different hue on the bottom of a decaying branch. And no, they are not edible. However, some crusts take on wild textures and colors whose evolutionary purpose defies explanation. If one dares to travel to the microscopic realms, they are greeted by truly bizarre structures like swollen oily cells, crystal-shrouded lances, and spiky clubs. To look into the microscope at a crust is to touch down on an alien planet with fractal geography and ornate constructions.

Morphological Definition

While crusts often speak for themselves, mycologists do have a technical definition for what constitutes a corticioid fungus. According to Larsson (2007), corticioid fungi are basidiomycetes with effused, resupinate basidiomata; a smooth, merulioid, or hydnoid hymenophore; and holobasidia. Let's break that down. Crusts belong to the class Agaricomycetes in the phylum Basidiomycota. These are fungi that produce spores on cells called basidia, primarily ones that do not have divisions (septa) — they're whole (holobasidia). Those Agaricomycetes that we call crusts have resupinate and effused basidiomata (fruiting bodies, or mushrooms), meaning they grow flat on the substrate and spread out, usually with no distinct shape, following the substrate until their growth is checked by the environment. The spore-bearing surface of this sheet-like fungus can be smooth, wrinkled, or with teeth ranging from almost imperceptible bumps to icicle-like projections. Now, relax your idea of what a crust is because there are exceptions to all the rules stated above. Some crusts do have defined shapes, some follow the substrate for a short period of time and then form cap-like structures, and others are not resupinate at all, growing straight from the ground like a vase.

Convergent Evolution of the Corticioid Form

Crusts are hard to pinpoint because they are a form group (defined by a similar morphology) rather than a monophyletic group (defined by a shared evolutionary history). In the past, all crust fungi were thought of as closely related species and were placed together in the family Corticiaceae — hence the name corticioid fungus, signifiying an affinity to fungi in the family Corticiaceae. Nowadays, we can sequence an organism's DNA to more accurately determine its relatedness to other species and this has drastically changed corticioid taxonomy. Crust fungi are actually distributed across most of the orders in the the class Agaricomycetes, intermixed with more popular mushrooms such as agarics, polypores, boletes, puffballs, and chanterelles. Related to each of these very different types of mushrooms are crusts that, while looking very similar to each other, are only distantly related.

Suillus acidus, a member of the order Boletales

BOLETALES

Serpula himantioides, a member of the order Boletales
Fomes excavatus, a member of the order Polyporales

POLYPORALES

Hydnophlebia chrysorhizaon, a member of teh order Polyporales
Russula sp., a member of the order Russulales

RUSSULALES

Aleurodiscus oakesii, a member of the order Russulales

Figure 1. Boletales, Polyporales, and Russulales are taxonomic orders that we typically associate with non-corticioid mushrooms (boletes, polypores, and agarics, respectively) but actually contain corticioid fungi. This shows that crust fungi are sometimes only distantly related, even if they look very similar.

All Agaricomycetes, in fact, may be derived from a crust-like ancestor. Over hundreds of millions of years, fungi have evolved extraordinary shapes, colors, and mechanisms of spore dispersal, but many have retained the crust form, and many others have converged upon this simple morphology. Crusts come and go again and again over evolutionary time like the kneading of dough. As a result, scientists recognize a very large number of crusts (thousands of species) spread across more than 282 genera and 41 families. Given the fact that we have documented less than 10% of all fungi and that relatively little attention has been devoted to crusts, the number of species of crust fungi is most definitely far greater.

Mycologists Wanted

Why do I collect crusts and why should you? As a mycologist, I am curious about all the members of the Kingdom Fungi. When you start foraging, you invariably run into crusts, and start to wonder, what are they and what are they doing? Not much accessible yet scientifically detailed information exists on the internet. Crusts are wildly understudied and underexplored. If you are intersted in discovering new species or having a large contribution to the documentation of the biogeography of fungi, whether as a citizen scientist or professional, crusts have a lot to offer.

Collecting Crusts

Unlike the meditative wanderings of mushroom hunters, crusts are most easily discovered by meticulously searching a single area with lots of fallen branches. While crust fungi might be visible on the tops of the branches, you'll find the greatest diveristy by picking the branches up and scanning their undersides. Log rolling is another fruitful method — you will be delighted by all the different organisms that grow and live on and under a large log, even when it is cold out. The living bark and dead wood of standing trees and shrubs are great places to find hardy species that don't mind drying out. More elusive species, like ectomycorrhizal crusts, grow on the bottom of leaf debris and conifer duff.

Some distinct crusts can be identified in the field by their macromorphology alone. Many, however, are impossible to identify to species without microscopic study or DNA sequencing. Regardless, succesful identification begins with close inspection of the crust and careful notetaking of its features.

References

Hibbett, D. S., Bauer, R., Binder, M., Giachini, A. J., Hosaka, K., Justo, A., … Thorn, R. G. (2014). Agaricomycetes. In D. J. McLaughlin & J. W. Spatafora (Eds.), Systematics and Evolution: Part A: Second Edition (pp. 379–429).

Larsson, K.-H. (2007). Re-thinking the classification of corticioid fungi. Mycological Research, 111(9), 1040–1063.