Blue Spot Knight
Tricholoma columbetta
Also called blue-spot knight, dove-coloured tricholoma, silky knight
A silky, pure white woodland knight whose cap and stipe base develop distinctive blue-green or pinkish spots and streaks with age. The white sinuate gills and complete absence of a volva place it firmly in Tricholoma.
Overview
The Blue Spot Knight (Tricholoma columbetta) is one of the handsomest white species of the genus. The Latin epithet refers to a dove, alluding to the smooth, silky, pale plumage-like cap surface. Fully grown fruitbodies are firm and substantial, with a slightly wavy, lobed cap margin and a solid tapering stipe.
Its trademark character is the appearance, in older or weathered material, of scattered blue-green, verdigris or occasionally pinkish spots and streaks on the cap margin and towards the stipe base. Not every fruitbody shows them, but when present they are unlike anything else in the white knights and immediately narrow the identification.
As in all Tricholoma, there is no ring and no volva, and the gills are notched where they meet the stipe. Recognising those two structural points is the essential first step before working through the white species, because several unrelated white woodland fungi that look similar from above are built quite differently at the stipe base.
How to identify it
Cap
4 to 10 cm across, convex then broadly flattened, often becoming irregular with a wavy or lobed margin. The surface is dry to slightly greasy in wet weather, silky-fibrillose and shining, pure white at first. With age it may be marked with pale blue-green, verdigris or rose-pink spots, particularly near the margin.
Gills
White and remaining white or barely cream, crowded, fairly narrow, and sinuate with a clear notch at the stipe. Gill edges may darken very slightly where bruised but do not turn brown.
Stipe
5 to 10 cm tall and 1 to 2 cm thick, solid, white, silky-fibrillose, usually tapering towards the base and sometimes slightly rooting. The base frequently shows blue-green or greenish-blue staining. There is no ring, and critically no sac-like volva or bulb sitting in a membranous cup.
Flesh & spore print
The flesh is firm and white throughout and does not change appreciably when cut. Odour is faint and slightly farinaceous. The spore print is white and the spores are smooth, hyaline and ellipsoid.
Where and when it grows
This species grows on soil in broadleaved and mixed woodland, most often with beech and birch, and also with oak, hornbeam and occasionally in mixed conifer stands. It shows a preference for acid to neutral soils, appearing on mossy banks, along woodland rides and in old coppice, either singly or in small scattered groups.
Fruiting runs from late summer into autumn, generally from August to October in temperate Europe. The species is widespread across Europe, with a scattered distribution that makes it locally frequent in suitable old woodland but rarely abundant.
Ecological role
Tricholoma columbetta is ectomycorrhizal, forming sheathing associations with the roots of its broadleaved hosts. The fungal mantle and Hartig net supply the tree with mineral nutrients and improve its access to soil moisture, while the fungus obtains all its carbon from the host. This is why the species is confined to established woodland and never appears on wood chips, compost or open grassland.
The blue-green pigmentation seen in older fruitbodies is thought to arise from oxidation of compounds in the surface tissues after damage or weathering, in much the same way that other fungi bruise blue or red. It appears progressively rather than instantly, so young pristine caps may be entirely white.
Similar species
Tricholoma album is a similar white woodland knight but has a duller, matt to slightly ochre-tinged cap, never shows blue-green spotting, and has an unpleasant sharp odour. Tricholoma stiparophyllum is likewise dull white with a strong odour and a more scurfy stipe apex apex. Tricholoma sciodes and other grey species are excluded at once by cap colour.
Most importantly, white Amanita species such as the Destroying Angel (Amanita virosa) are built quite differently: their gills are free, not notched, they carry a membranous skirt-like ring on the stipe, and the base sits in a conspicuous white sac-like volva. Digging the whole base out intact settles the question. Leucoagaricus and Leucoagaricus leucothites also have free gills plus a movable ring, while white Clitocybe species have decurrent gills that run down the stipe.