"Asterophora, the piggyback mushrooms" by Sue Lancelle

Mycotrophs are fungi that grow on other fungi, either as saprobes or parasites. Many of these are parasites that you might be familiar with: the various Hypomyces that form a crust-like covering on many different mushrooms; Entoloma arbortivum, the “aborted entoloma” that parasitizes an Armillaria; Pseudoboletus parasiticus, parasitizing the earthball Scleroderma citrinum, and various “cordyceps” that parasitize truffles. There are many more; see Michael Kuo’s “Key to 25 Mushroom Eating Mushrooms and Fungi.” However, there is one particularly interesting genus of parasitic mushrooms that we only occasionally run across, and those are species of Asterophora, the “piggyback mushrooms.”

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Figure 1. Illustration of A. lycoperdoides by Oscar Brefeld. The parasite is growing on Russula nigricans. Note the detailed drawings of chlamydospores, especially just under the right side of the Russula cap.

Currently there are two species of Asterophora recognized from North America: A. lycoperdoides   A. parasitica. These are small (caps typically 1-2 cm when mature) mushrooms that parasitize species of Russula and occasionally Lactarius. Last summer and fall we were observing more of both of these species than is usual; perhaps it had something to do with the very wet weather we had. What makes these species especially interesting is that they rarely produce basidiospores, the reproductive spores produced by basidia in the gills and pores of Basidiomycetes. Species of Asterophora reproduce mainly asexually by the production of chlamydospores, thick-walled cells produced directly by hyphae. 

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Figure 2: Illustration of A. parasitica by Oscar Brefeld, showing the more highly developed gills of this species, as well as the development of the smooth chlamydospores (lower right) that occurs within the cap tissue.

German mycologist Oscar Brefeld  repeated and confirmed earlier experiments that showed that mushroom caps parasitized with Asterophora broke down more slowly than those that were not parasitized. Presumably, this effect allows the parasite to more efficiently absorb nutrients from its victim. The chlamydospores of A. lycoperdoides are formed on the surface of the cap. As they mature, they form a powdery tan cap surface of spores that are then released into the wind, hence the common name “powdery piggyback mushroom.”

The other species that we encounter, A. parasitica, is commonly known as the “silky piggyback mushroom.” This species also reproduces by chlamydospores, but they are not formed on the surface of the cap, which maintains its silky texture through maturity. Rather, the chlamydospores form in the cap tissue itself, and sometimes on the gills, but again, they are formed asexually by hyphae rather than through the normal sexual reproductive process that results in basidiospores. Chlamydospores of A. parasitica are smooth, and easy to distinguish from A. lycoperdoides. Gills are often thick and more well developed than in A. lycoperdoides, but as in that species, they rarely produce basidia.

To look for these interesting little mushrooms, it helps to pay attention to what might at first glance just look like a brown or black blob of disintegrating mushroom in late summer to early fall. Poke around a little more closely, and you may be lucky enough to find one of our piggyback mushrooms!    

References and Resources

Australian National Botanical Gardens. Case Studies: Fungi on Fungi – Asterophora. https://www.anbg.gov.au/fungi/case-studies/asterophora.html. Accessed Feb. 23, 201 9.

Brefeld, Oscar. 1 877. Untersuchungen aus dem Gesammtgebiete der Mykologie. Verlag Von Arthur Felix, Leipzig. Images used are from Part 8, Basidiomyceten III. Available through the New York Botanical Garden: https://www.biodiversitylibrary.org/bibliography/3887#/summary Accessed Feb 23, 201 9.

Kuo, M. 2006. Asterophora lycoperdoides. http://www.mushroomexpert.com/asterophora_lycoperdoides.html. Accessed Feb. 23, 201 9.

Kuo, M. 2006. Key to 25 mushroom-eating mushrooms and fungi (mycotrophs). http://www.mushroomexpert.com/mycotrophs.html. Accessed Feb. 23, 201 9.

Palmer, J. and Volk, T. 2005. Asterophora lycoperdoides, the star bearing powder cap mushroom. https://botit.botany.wisc.edu/toms_fungi/dec2005.html.
Accessed Feb. 23, 201 9.