Aleurodiscus oakesii Identification Guide

The First Impression

Aleurodiscus oakesii is not found by scanning the ground. You find it by reading bark. Walking through a mature oak wood in eastern North America, you notice that a large white oak has pale, smooth, slightly sunken areas on its trunk, sometimes the size of a hand, sometimes running together into patches a metre across. Where the surrounding bark is deeply furrowed and dark, these areas look scrubbed clean, greyish and flat. That contrast is the classic sign of what is often called smooth patch.

Walk up to the trunk and put your eye within 20 cm of the bark, ideally with a hand lens. Scattered over the patch and concentrated around its margins are minute pale discs, 1-3 mm across, looking at first like flecks of dried paint or lichen. Those discs are the fungus. Damp conditions make them swell, pale and become obvious; in dry spells they shrink to hard grey specks that are easy to walk past.

Reading the Fruit Body

There is no cap and no stipe here, so the reading sequence is different from an agaric. Work through the following in order.

Outline and attachment. Individual fruit bodies begin as small rounded cushions that flatten and expand into discs or shallow cups. They are sessile or narrowly attached at a central point, sitting directly on the bark surface. With age neighbouring discs fuse into irregular, patchy crusts several centimetres long that follow the contours of the bark.

The fertile surface. The upper or outward-facing side is the hymenium. It is smooth to slightly uneven or wrinkled, matt rather than shiny, and coloured whitish, pale buff, grey-cream or faintly pinkish-grey when fresh and moist. There are no pores, teeth, gills or veins of any kind, and that plainness is diagnostic at this size.

The margin. Under a lens the rim is finely fringed, minutely downy or frosted, and usually slightly raised or inrolled, giving the disc a shallow saucer shape. This distinct margin is a good separator from the truly flat film-like crust fungi.

Texture and hydration behaviour. Fresh material is firm, slightly waxy and tough, not gelatinous and not brittle. As it dries the disc contracts, curls at the edges, hardens and darkens to grey-brown or drab ochre. After rain it rehydrates within hours and resumes its pale, plump appearance. Testing this cycle on a marked patch across a wet and a dry day is one of the most convincing field confirmations available for a crust fungus.

Taking a Spore Deposit

A conventional spore print is awkward but not impossible. Cut a small chip of bark carrying several fresh, well-hydrated discs. In a covered container lined with damp paper, suspend or prop the chip with the hymenium facing down onto a clean glass slide, leaving a millimetre or two of clearance. Leave it overnight in a cool place. The deposit is white and often very sparse, so tilt the slide against a dark background to see it.

Microscopy is what really settles the identification, and this species rewards it. The basidiospores are enormous by any standard, roughly 20-28 by 16-22 micrometres, broadly ellipsoid to almost globose, and ornamented with fine warts or short spines. Mounted in Melzer's reagent the spore wall and ornamentation react amyloid, staining blue-grey. The hymenium also contains acanthophyses, slender sterile cells whose upper part bears short finger-like spiny projections, alongside very large club-shaped basidia. Giant warted amyloid spores plus acanthophyses on white oak bark is effectively a complete determination.

Substrate, Season and Ecology

The substrate is the identification. Aleurodiscus oakesii occupies the outer, dead corky layers of the bark of living standing trees, most typically white oak and its close relatives, and it is also recorded from other broadleaved trees including hickory and maple. It does not decay the sapwood or heartwood, and its activity is confined to the rhytidome, the accumulated dead outer bark. As those layers break down and flake away, the smooth depressed patch is revealed, which is why the patches and the discs are found together.

Fruit bodies are perennial in effect, persisting on the bark and reviving repeatedly, but they are most conspicuous during cool damp periods from autumn through to spring, and after prolonged rain at any season. Look on the lower and middle trunk of mature trees, especially on the shaded and wetter side, and on large limbs of old open-grown oaks.

Separating the Look-alikes

Aleurodiscus amorphus is the classic congener and is easy to separate on substrate and colour: it grows on the bark of firs and other conifers, and its discs are distinctly orange to pinkish-orange with a conspicuous white fringed rim.

Dendrothele species also live on living bark and also have very large spores, but they form thin, chalky-white, evenly spread films without discrete disc-shaped fruit bodies or a raised fringed margin. Run a fingernail across the surface: Dendrothele is a smooth coating, Aleurodiscus oakesii is a scatter of individual saucers.

Peniophora and other resupinate crusts form smooth, waxy or slightly cracked sheets, often pinkish, lilac or ochre, and they occupy dead attached branches and fallen wood rather than the trunk bark of living trees.

Stereum and Xylobolus species are far larger, form brackets or extensive tough crusts with brown or zoned upper surfaces, and grow on dead wood or on exposed decayed heartwood, not on intact living bark.

Cyphelloid fungi such as Merismodes and Lachnella produce small cups too, but they are hairy or shaggy on the outside and typically hang from small dead twigs.

Crustose lichens are the most frequent confusion because they occupy the same trunks. Lichen thalli are firmly bonded to the bark, often greenish or blue-grey, and their apothecia have a coloured disc set in a rim of thallus tissue. Scrape gently: a lichen crust is brittle and mineral-like, while the Aleurodiscus disc is a discrete, tough, fleshy structure that can be prised off whole and that visibly swells when wetted.

A Working Sequence

Locate a smooth patch on a mature oak, then hunt the patch margin with a lens for pale 1-3 mm discs with fringed inrolled rims. Wet one with a drop of water and watch it plump up. Confirm there are no pores, veins or hairs. Take a chip home, deposit spores overnight, and check for very large ornamented amyloid spores and acanthophyses. Substrate, disc form and spore size together give a firm record.