Armillaria borealis, also known as Northern Honey Fungus, is a medium-sized agaric that thrives in the cooler climates of the northern hemisphere. This species, part of the Armillaria genus, is characterized by its honey-coloured cap and clustered growth pattern, often found on the decaying roots or lower trunks of trees. Notably, it is a parasitic fungus, capable of causing white rot in its host trees, making it a key player in forest ecosystems. Though it contributes to the decomposition process, Armillaria borealis can also be harmful to forests, as it is a known tree pathogen. Despite its somewhat destructive nature, it is a fascinating mushroom.

Cap hemispherical, eventually flattening, honey-yellow, darker in the centre with a dense covering of brown scales, translucent-striate when moist and especially as it ages along the edges. The flesh is white, thin and firm. Gills adnate or weakly decurrent, white, later becoming yellowish with brown spots. In young mushrooms, the gills are covered by a white veil. Once the veil detaches from the cap's edge, it remains hanging as a ring around the stem. Stem more and less equal but it can also be club-shaped and quite thick at the base. It is palest at the apex, yellowish-brown at the base, and the surface has persistent yellowish woolly scales. The upper part of the stem features a strong, cottony ring, which is speckled brown on the underside. Spore print very pale cream.

Microscopic Features: The spores are ellipsoidal, smooth, typically contain guttules, measure 7.5-10 x 5-7 µm, and are amyloid in reaction.

Armillaria borealis on the First Nature Web site.