That weird fungus I found last year in June turns out to be two fungi. It, or rather they, were growing from a mossy decomposed log in my woods. The whole thing looked a bit like a chunk of old foam rubber. Not being an expert in fungi I started searching images with queries like “globular fungus” and “tuberous fungus”. This soon led me to truffles and their kin. Then, after seeing cross sections of these globular fungi with their layered interiors, I was steered towards the truffle relatives Geopora and Hydnocystis. By that time I was way off the path to the right identification especially after I saw an exterior shot and cross-section of Geopora cooperi which has a fuzzy exterior and distinctly convoluted interior. (See more photos of Geopora and Hydnocystis here on the Asociacion Vallisoletana de Micologia web site. But G. cooperi, as far as is known, does not grow in Minnesota. Some species of Hydnocystis are reported to grow here, though. I was close, in the right family (Pezizales), but was looking at the wrong genera and missing a very important microscopic feature.
One night I was browsing the internet about another fungus (Datronia scutellata) and came across a very informative site called Weird and Wonderful Wild Mushrooms. Going through the archives I happened upon a post on false morels. The picture of Gyromitra gigas in cross section reminded me of what I had found last year. After some communication with the blogger it turned out that this foam rubber fungus is neither Geopora or Hydnocystis but a Gyromitra, most likely G. esculenta, very common here, that has been infected by another fungus called Sphaeronaemella helvellae. The fuzz over the fungus surface was S. helvellae and not, as I first thought, a coating soft hairs on its surface. So, this is another infected mushroom!
Infection by S. helvellae does more than coat the Gyromitra with fuzz. It also deforms it and this globular deformity and the layered interior led me in the direction of truffle relatives. There are many parasitic fungi that infect other fungi. One genus, Hypomyces, contains some 53 different species that infect a huge range of gilled mushrooms including Russula, Lactarius, Suillus, and Amanita. The mushroom Psathyrella epimyces infects Coprinus, the inky caps, and there are several species of Cordyceps that infect truffles.