Heterobasidion annosum

A polypore associated with conifers, especially spruce (Picea).

Dimitic hyphal system: skeletal hyphae thick-walled, 4-7 µm wide, slightly dextrinoid in Melzer's reagent; generative hyphae thin- to moderately thick-walled, 2-4 µm wide, hyaline, with simple septa (clampless).

Cystidia absent.

Basidia clavate, 17-30 µm long and 7-8 µm wide, without basal clamps, 4-spored.

Spores subglobose to ovoid, thin-walled and very minutely verruculose, with a prominent eccentric apiculus at one end, vacuolar internal content, virtually negative in Melzer's reagent (or at most very weakly and insignificantly dextrinoid).