A ubiquitous pathogenic fungus

So far it's been a pretty bleak fall for mushroom photography in the Lake States region. Rainfall has been short across northern Minnesota, Wisconsin, and the Upper Peninsula of Michigan, and even with the recent rain we've had, the fall fungi just haven't been emerging yet.

This little group of inocuous-looking 4" mushrooms are the sporocarps of a giant among fungi. This is an Armillaria species, most likely A. ostoyae. It is a close relative of the "Humongous Fungus" first identified near Crystal Falls, MI by a Forest Pathologist at Michigan Technical University. Armillaria species are parasites on over 1000 hosts, and the collective species may comprise one of the most ubiquitous genera of any kind in the terrestrial landscape.

Armillaria species are commonly called "honey mushrooms" because of the color of their caps. Most have an "annulus," a ring around the stem that once served as a veil to protect the gills of the fungus as it matured. Most or all of them are edible (standard disclaimer applies, of course), and some people find them very good. They usually fruit prolifically in the fall, especially near dead and declining trees. Yes, there often is a cause-and-effect relationship between the decline and the presence of mushrooms, but Armillaria fruits on decaying wood of trees killed by almost any other agent.

For an interesting look at another aspect of Armillaria biology, check out Tom Volk's fungus of the month for Sept. 2006, the apparently misnamed Entoloma abortivum.