Armillaria bulbosa

Suprapellis formed by filamentous hyphae, fairly broad, little or not at all pigmented, with numerous septa and without clamp connections.

Lamellar edge with some terminal, sinuous hair-like hyphae projecting above the level of the basidia.

Basidia cylindrical to clavate and 4-spored.

Spores ellipsoidal, smooth-walled, with granular content inside and inamyloid in Melzer's reagent.

Long considered a simple variety of the abundant Armillaria mellea, which has very similar microscopy, from which it is distinguished by its solitary fruiting habit and bulbous stipe base, a feature that gives rise to its specific epithet.