Powdery Brittlegill
Russula parazurea
Also called powdered brittlegill
A medium-sized brittlegill with a matt, slate-blue to grey-green cap covered in a fine frosted bloom. It is common with oak in parks, avenues and open woodland.
Overview
The Powdery Brittlegill, Russula parazurea, takes its English name from the delicate pruinose bloom that covers the cap surface, which looks as if the mushroom has been lightly dusted with flour or fine frost. Rubbed with a fingertip, the bloom wipes away to reveal a darker, smoother skin beneath.
It belongs to a difficult group of grey, blue-grey and green-capped brittlegills that includes Russula grisea, Russula ionochlora and Russula cyanoxantha. Within that group, R. parazurea is distinguished by the combination of its matt powdered cap, its cream spore deposit and its typically brittle gills.
The species is one of the most reliably urban brittlegills of the genus, appearing along tree-lined avenues, in city parks, on churchyard banks and beside footpaths wherever mature oaks or limes are present. This tolerance of disturbed, compacted ground makes it a familiar find even where the wider fungal flora is thin.
How to identify it
Cap
Usually 4-8 cm across, convex then flattened and finally shallowly depressed. The surface is dry and matt with a distinctive fine powdery or frosted bloom, especially towards the centre. Colours range through slate-blue, blue-grey, grey-green, olive and dull lilac-grey, frequently mixed, and the margin is often noticeably paler, sometimes almost buff. The skin peels roughly half way to the centre. The margin becomes faintly grooved only in old specimens.
Gills
Whitish at first, soon a definite cream, moderately crowded, adnexed and brittle. Forking near the stipe attachment is common, and the gill edges are entire. With age the gills may become spotted ochre where damaged.
Stipe
3-6 cm long, 1-2 cm thick, white and smooth, sometimes taking on greyish or faintly ochre tones with age or handling. It is cylindrical or slightly narrowed at the base, firm at first and becoming softly stuffed, without ring or volva.
Flesh & spore print
The flesh is white and unchanging or greying only slightly, firm in young caps. The spore print is cream, a distinctly deeper shade than the near-white deposits of the Charcoal Burner group, and this difference is one of the more useful separations. Spores are ellipsoid with isolated to partly connected warts.
Where and when it grows
Russula parazurea is mycorrhizal with broadleaf trees, most often oak, but also lime, beech, birch and hornbeam. It prefers well-drained neutral to slightly basic soils and is very frequently found in artificial or heavily managed settings — park lawns beneath scattered oaks, tree-lined roads, cemetery grass, woodland edges and grassy rides.
Fruiting spans late June to October, with the main flush in August and September. It is common and widespread throughout Europe and reported across temperate Asia; comparable grey-capped species occupy the same niche in North American deciduous forest.
Ecological role
As an ectomycorrhizal fungus, this species lives in intimate partnership with the fine roots of its host trees, forming a sheath of fungal tissue around the root tip and a network of hyphae between the outer root cells. Water and mineral nutrients scavenged from a large soil volume flow to the tree, while carbon compounds fixed in the leaves flow back to the fungus.
The tolerance of R. parazurea for compacted, disturbed and moderately alkaline soils makes it one of the more resilient mycorrhizal partners of urban and parkland trees, where soil conditions exclude many woodland fungi. Its fruit bodies are short-lived and are rapidly consumed by slugs and fly larvae, recycling their nutrients into the surface soil.
Similar species
The Charcoal Burner (Russula cyanoxantha) is larger, has a greasy rather than powdery cap, gills that flex like soft plastic instead of shattering, and a white spore print. Russula grisea is bigger and more lilac-grey, with a smoother, only faintly pruinose cap and a slightly deeper ochre-cream spore deposit.
Russula ionochlora is smaller and more delicate, with a cap mixing green and lilac tones over a paler ground and whiter gills. Russula azurea has a similar frosted look but is a conifer species of spruce woods with a whiter spore deposit and a more distinctly blue-lilac cap. Judging the cap bloom, the brittleness of the gills and the depth of the spore-print colour together will separate these species.