Aleurodiscus oakesii

Aleurodiscus oakesii is a small corticioid fungus in the family Stereaceae, the same broad group that contains the leathery Stereum crusts. Rather than decaying wood, it lives on and consumes only the dead outer bark of standing trees, which places it in an unusual niche among crust fungi.

It is best known in eastern North America, where it produces the condition arborists call smooth patch or bark patch on white oak and related species. The fungus digests the corky outer bark layers, and as those layers slough away the trunk is left with irregular, smooth, pale grey areas that contrast sharply with the surrounding ridged bark.

The fruiting bodies themselves are inconspicuous: small, pale, saucer-shaped discs scattered or crowded across the smooth patch, easily mistaken for lichen or weathering until examined closely.

Fruiting body

Individual fruitbodies are 1–10 mm across, at first shallowly cup-shaped or saucer-like with an inrolled edge, later flattening into irregular discs that may fuse into small colonies. They are leathery to slightly waxy when fresh and hard and brittle when dry.

Fertile surface

The upper (spore-bearing) face is smooth to minutely uneven and whitish, pale grey or buff, sometimes with a faint pinkish or cream tinge. There are no gills, teeth or pores. The outer face and margin are paler and finely felted or minutely hairy.

The bark patch

The most conspicuous character is not the fruitbody but the patch: irregular, often rounded to oblong areas of trunk where the rough outer bark has fallen away, leaving a smooth, slightly sunken, light grey surface. Patches can be a few centimetres to more than a metre across and often coalesce over old trunks.

Flesh and spore print

The context is thin, tough and pale. The spore deposit is white; spores are notably large and ornamented, and stain blue-grey in iodine solutions, which is a standard laboratory confirmation for the genus.

It grows on the bark of living hardwood trees, with a strong preference for white oak (Quercus alba) and other members of the white oak group; it is also recorded from other oaks, hickory, maple, elm and hackberry. Colonisation typically begins on mature trunks and major limbs where the outer bark is thick.

The fungus is common through eastern and central North America, with related Aleurodiscus species occurring across Europe and Asia on other hosts. Fruitbodies are produced mainly in cool, damp weather from autumn through spring and remain visible, if shrivelled, all year; the smooth patches persist indefinitely.

This is a saprotroph with an unusually narrow diet: it decomposes the dead corky tissue of the outer bark (the rhytidome) and does not enter the living cambium, sapwood or heartwood. Because the outer bark is already dead tissue, the tree's structural wood and conducting tissues are unaffected, and the patches simply mark where old bark has been consumed and shed.

Spores released from the discs are wind-dispersed to other bark surfaces, where they germinate in the fissures of thick, mature bark. Growth is slow, and a large smooth patch may represent many years of colonisation by a single mycelium, expanding outwards as fresh bark layers are reached.

Crustose lichens are the commonest confusion, since they also form pale grey patches on trunks; lichens have a continuous thallus with reproductive discs embedded in it and often show a distinct dark rim or powdery soredia, whereas Aleurodiscus oakesii produces free-standing leathery discs sitting on exposed, smoothed bark. Stereum species are much larger, form fan-shaped brackets with a hairy zoned upper surface, and grow on dead wood rather than living bark.

Other Aleurodiscus species, such as the orange-tinged A. amorphus on conifer bark, differ in colour and host. Frost cracks, sunscald and mechanical bark loss can also leave smooth trunk areas, but these lack the scattered small pale discs and usually show torn or callused edges rather than a gradual boundary of flaking bark.