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The Genus Armillaria  

[ Basidiomycota > Agaricales > Physalaciaceae . . . ]

by Michael Kuo

The genus Armillaria contains wood-rotting gilled mushrooms with white spore prints and gills that are attached to the stem or run down it. Most of the species have a partial veil, but the veil can manifest in several different forms—from cob-webby ring zones to full-blown rings. Roughly a dozen species occur in North America. Many of these are parasitic, and many form visible black rhizomorphs in the wood; others are apparently benign saprobes and do not form conspicuous rhizomorphs.

The species in Armillaria are well-studied and supported by all four "legs" of the species definition stool: ecological, morphological, biological, and phylogenetic separation. In other words, DNA studies, mating studies, and traditional morphology (in most cases) agree on the species, which are also limited to specific environmental niches. This kind of species-concept confluence is not the norm in mycology, so enjoy this genus if you are someone who dislikes confusion!

Identification of Armillaria species can often be accomplished with close attention to physical features and collection location. However, a few species pairs, in limited geographic areas where they overlap, are morphologically "cryptic" and can only reliably be separated with mating or DNA study (see the key below). Microscopic examination is generally not useful in North American Armillaria identification; differences in spore size, cheilocystidia, and pileipellis are not significant between species—and while Armillaria mellea is the only species lacking basal clamps on its basidia, it is easily separated with macroscopic features.


Armillaria gallica

Armillaria mellea

Armillaria mellea

Key to 9 Species of Armillaria in North America

1.Stem without a ring or cobweb-like ring zone.

1.Stem with a ring or ring zone.

2.Mature cap with prominent scales.

2.Mature cap smooth, with scattered fine hairs, or with tiny scales, but without prominent scales.

3.Found on hardwoods; distributed from the Appalachian Mountains eastward; rare; benign.

Armillaria gemina
(see Armillaria solidipes)

3.Found primarily on conifers; widely distributed in northern North America; common; parasitic.

4.Found at high elevations in interior forests with fir; stem base swollen.

Armillaria altimontana
(see Armillaria solidipes)

4.Found in a variety of coniferous forests; stem bases equal or narrowed.

5.Cap color orange-brown; stem dark and colored like the cap; growing on hardwoods (especially species of Alnus) in western North America.

Armillaria nabsnona

5.Not completely as above.

6.Stem with a thick, membranous ring; stem bases pointed due to growth habit in dense clusters; basidia without basal clamps.

6.Ring ephemeral or cobwebby; stem bases not pointed; (at least some) basidia basally clamped.

7.Pathogenic and parasitic, invading the wood of hardwoods (in the northeast) and conifers (in the Pacific Northwest) with black rhizomorphs; mushrooms typically fruiting from visible wood.

Armillaria sinapina

7.Harmlessly saprobic on hardwood (and occasionally conifer) roots east of the Rocky Mountains; not displaying prominent black rhizomorphs in the host wood; mushrooms often appearing terrestrial, though occasionally fruiting from visible wood.

8.Mating in the laboratory with previously identified isolates of Armillaria gallica; widely distributed east of the Rocky Mountains.

8.Mating in the laboratory with previously identified isolates of Armillaria calvescens; northeastern in distribution.
Armillaria calvescens
(see Armillaria gallica)


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Volk, T. J. (2005). Key to North American Armillaria species. Retrieved May 18, 2017 from the Tom Volk's Fungi Web site:

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Kuo, M. (2017, September). The genus Armillaria. Retrieved from the MushroomExpert.Com Web site:

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