Elm Leech

Description

The elm leech is a pale, creamy-white mushroom with a cap 6-15 cm across, dry and smooth, set on a thick, white, almost central stem without a ring. The gills are white, crowded and only slightly decurrent on the stem, which distinguishes it from similar oyster mushrooms.

It grows from July to November on broadleaved trees, most often on elms, often high up on the trunk, in wounds and scars, in forests, parks and along roads. It is rare in Poland, so it is often found only singly or in small clusters.

Because it can be confused with oyster mushrooms and other white gilled mushrooms, the full set of features counts: weakly decurrent gills, no ring and the substrate. If you are not sure of the species, do not pick or eat it.

Identification features

Cap

Diameter 6-15 cm (exceptionally up to 30 cm), convex with a broad margin (according to some descriptions the margin is rolled under; larger caps may be concave). Surface dry, smooth, whitish to cream in colour; in older specimens it may crack into areolae.

Hymenophore (underside of the cap)

Gills adnate or slightly decurrent, broad, crowded, white (in oyster mushrooms Pleurotus the gills run deeply down the stem).

Stem

Height 6-13 cm, thickness 2-3 cm, cylindrical, smooth or finely fibrous, white, attached almost centrally; no ring.

Flesh

Smell sour, slight or sometimes faintly mealy; taste mild. No data on colour change.

Spore print and spores

Spore print cream, according to some descriptions white to pale cream. Spores 3.5-6.5 × 3-5 µm or about 5-7 µm (Redhead's original description), subglobose to ellipsoid, non-amyloid. Data on spore size are inconsistent.

Habitat and distribution

Ecology. A tree-dwelling parasite and saprotroph (unclear whether it feeds on living or dead tissue, although it occurs on living trees). Causes brown rot of damaged places, but also secretes laccase that breaks down lignin. Grows high up in wounds and scars of trees. Most often on elms (Ulmus laevis, U. minor), also on sycamore, horse chestnut, aspen, birch, beech, oak, linden and box elder.

Distribution. Forests, parks, botanical gardens and roads in the temperate zone of North America, Europe and Asia; relatively rare compared with oyster mushrooms. In Poland on the 2006 Red List with status V (vulnerable to extinction). Not legally protected.

Fruiting. From July to November.

What can it be confused with?

• Pleurotus spp. (oyster mushrooms) — Gills running deeply down the stem; oyster mushrooms cause white rot, the elm leech brown rot.
• Hypsizygus tessulatus — More often grows in larger clusters.
• Ossicaulis lignatilis — Smaller, grows out of cavities in wood.
• Pleurotus levis — Gills decurrent.

Never rely on a single feature. If you are not sure — do not pick it; show the mushroom to a qualified mushroom expert.

Interesting facts

• The genus name Hypsizygus means "high" and "yoke", which refers to its growth high up on the trunk of a tree; the epithet ulmarius comes from the elm (Ulmus).
• Over the years the species changed genus: Agaricus → Pleurotus → Lyophyllum → Hypsizygus (1984).
• It produces laccase, an enzyme that breaks down lignin and dyes, studied in mycoremediation.
• Until 2003 Polish literature gave it under several different common names, including "bedłka wiązowa" and "boczniak wiązowy".