What You Should Know
Armillaria borealis is a species of fungus in the family Physalacriaceae. The cap is orange-brown, sometimes with an olive tinge, darker in the center. The stem is similar to the cap color and the gills are adnate white.
Grows primarily in Europe and is widely distributed across Southern Scandinavia, Northern France, Netherlands, and Scotland. It was also reported in China, Siberia, and on mountains of Iran.
It is edible but requires longer heat treatment (at least 30 minutes) because they contain thermolabile substances that can cause digestive problems. Consider collecting only caps on the youngest trees. If eaten raw it is slightly poisonous.
These species cause shoestring root rot, which leads to significant losses in forest areas or woody plants, including forests, parks, or vineyards, among others, mostly in the temperate zone. The infection is usually characterized by the presence of rhizomorphs and mycelial mats between the bark and cambium layer of the host root.
Other names: Northern Honey Fungus, Noordelijke honingzwam (Netherlands), Václavka Severská (Czech Republic), Nördlicher Hallimasch (German).
Armillaria borealis Mushroom Identification
The cap is 30–80 (100) mm wide, at first convex, later almost flat, usually quite light, ochre-brown, orange-brown or yellowish, covered with light tawny or ocher scales on the crown, soon almost glabrous at the margin. The edge of the cap is raised when young and connected to the stem by a whitish veil.
The attached or weakly descending gills are crowded, initially almost white, gradually becoming cream, orange at maturity, and scattered with brown rust-like spots.
The stem is cylindrical, often club-shaped, 60–100 x 5–15 mm, whitish under the cap, sometimes with irregular whitish bands or rings from the remains of the veil.
The flesh is whitish, unchanging when cut, hard. The smell is indistinct. The taste is slightly astringent.
Oval, elliptical, according to Laessoe and Petersen (2019) or Knudsen, Vesterholt et al. (2018) 7–8.5 x 4.5–5.5 µm, according to Antonín and Tomšovský (2010) 7–12 x 6–8.5 µm (this size is closer to our observation), smooth, colorless.
It grows abundantly at higher altitudes, usually in rich bunches on tree stumps and living trunks of deciduous trees, mainly birches, but also conifers.
Summer to Autumn.
Armillaria borealis Look-Alikes
Grows in warmer areas and has a yellow cap and a yellow-lined ring.
Grows on hardwood trees and is similar in color and covered with scales. It retains curled edges, gills turn a uniform rust-brown color and has a radish-like smell and taste.
Armillaria borealis Taxonomy and Etymology
This forest fungus was identified in France as a unique species, Armillaria borealis, after it was published in 1982 by German mycologist Helga Marxmüller and Finnish mycologist Kari Korhonen.
The specific epithet Borealis comes from the Greek noun Boréas, which means "of the north".
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