Meripilus giganteus

The basidiocarps consist of numerous rosette-like flattened fan-shaped pilei; they are typically 50–150 centimetres (20–59 in), rarely 250 centimetres (98 in) in diameter and 10–50 centimetres (3.9–19.7 in), rarely 90 centimetres (35 in) high. The individual caps, up to 10–50 centimetres (3.9–19.7 in), rarely 90 centimetres (35 in) in diameter and 1–5 centimetres (0.39–1.97 in) thick, arise from a common basal stem.

An older specimen of M. giganteus with brown, zonate pilei.

The weight is 10–50 kilograms (22–110 lb), but the heaviest specimen can reach 90 kilograms (200 lb).

The color of the cap surface is pale tan to dull chestnut brown in young specimens but darkens in age to become concentric zones (zonate) of various shades of brown.

The surface is also finely fibrillose with tiny scales (squamules). There are 3 to 6 pores per millimeter on the underside; the pore surface bruises brown and black, helping to distinguish it from the similar species Grifola frondosa.

Infection of a tree is often through a dead tap root, and decay is largely restricted to roots, and then mainly on the underside. Infected trees often show thinning of the outer crown due to impaired root function. Tree failure is due to brittle fracture of degraded lateral roots.[1]

Microscopic features

Spores are roughly spherical to ovoid or ellipsoid in shape, with typical dimensions of 6–6.5 × 5.5–6 μm. Under a microscope, they appear translucent (hyaline), smooth, and nonamyloid, meaning that they do not absorb stain from Melzer's reagent.[2] The basidia—the spore-bearing cells—are club-shaped, 4-spored, and are 22–40 by 7–8 μm.[3]

Polypore fungi may be further distinguished by the type of hyphae that makes up their fruiting body. M. giganteus has a so-called monomitic hyphal system, as its fruiting body is composed of only vegetative hyphae.[4]

The giant polypore was previously considered inedible, due to its very coarse flesh and mildly acidic taste,[5] but more recent sources list it as edible.[6][7] Younger specimens may be more palatable; one author notes that it is "eaten in Japan".[4] Also, it may be mistakenly consumed because of its resemblance with the edible species commonly known as Hen of the Woods (Grifola frondosa) which is regarded as much better tasting.[7]

Although usually found on tree trunks, it is not uncommon to find it away from the tree

This mushroom can be found growing on hardwoods, more rarely on conifers. According to Ryvarden and Gilbertson in their monograph on the polypores of Europe, M. giganteus grows especially on Quercus and Fagus tree species, but it has also been collected on the hardwoods Acer, Aesculus, Alnus, Betula, Castanea, Celtis, Corylus, Eucalyptus, Laurus, Myrica, Persea, Pittosporum, Platanus, Populus, Prunus, Pyrus, Tilia, Ulmus; it has also been found growing on the coniferous species Abies, Larix, and Pinus.[3]

M. giganteus has a circumboreal distribution in the northern hemisphere.[8] It has been collected from Europe, Scandinavia, the area formerly known as the USSR, Iran and Turkey.[9] Although many field guides list it as occurring in North America, this is due to confusion with the related M. sumstinei; M. giganteus is not found in North America.[10][2] A study of the frequency of occurrence of wood-decay fungi on street trees and park trees in Hamburg, Germany found that M. giganteus was the most common species.[11]

The methanol extract from this mushroom has been shown to contain compounds that are significantly cytotoxic to the mouse cancer cell line 3LL.[12]

The polypore fungus Grifola frondosa is similar in overall appearance, but may be distinguished by its more greyish cap, and larger pores.[3] Bondarzewia berkeleyi or "Berkeley's polypore" is often confused with M. giganteus (or M. sumstinei) in eastern North America but can be distinguished by its lack of black-bruising and much larger pores.[13]

• Index Fungorum synonyms
• USDA ARS Fungal Database
• Meripilus: A new perspective
• Mushroom Expert
• Fungi on Wood