Kingdom: Fungi
Division: Basidiomycota
Class: Agaricomycetes
Order: Agaricales
Family: Strophariaceae (formerly Bolbitiaceae)
Genus: Agrocybe
Species: A. parasitica
Binomial name: Agrocybe parasitica
Synonyms: Pholiota pudica, Agaricus pudicus, Agrocybe cylindracea
Common name: Poplar Mushroom

Agrocybe parasitica is typically considered a native heart rot fungus of hardwoods. It occurs on a wide range of both native and introduced hardwood tree species (elms, poplars) but not on conifers. It is usually seen growing solitary or in groups during summer to Autumn. It is common throughout New Zealand. Agrocybe parasitica can grow high up the trunks of trees.
Agrocybe parasitica can grow to a very large size with a cap up to 300 mm wide.
Once the fruiting body is mature they are easy to identify because the membrane that covered the gills forms a dark skirt on the stalk after it had broken away from the cap. The spores are a dark brown.

Juvenile Agrocybe parasitica
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The underside of a young mushroom showing the complete membrane covering the gills.
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An adult cap of an Agrocybe parasitica growing on Melicytus ramiflorus (Mahoe)1-Agrocybe parasitica 1 .jpg 

An adult cap with gill membrane still intact.
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The underside of the cap showing the gill membrane breaking away.
3-Agrocybe parasitica 4 .jpg

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Photo showing the identifying remnants of the dark skirt on the stalk.
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Thanks to Wikipedia for text and information: