Don't mistake the poisonous Cortinarius Australiensis for an edible horse mushroom.

The Horse Mushroom, Agaricus  arvensis  is  a large field-type mushroom that is considered good eating  although it can be a little tough.

The Poisonous Cortinarius australiensis can look quite like an edible horse mushroom.

However, in Australia foragers should take care  not to confuse it with the large and poisonous Cortinarius australiensis.  From a distance they look very similar.  

Both are

•     about the same size

•     fleshy

•     have hints of yellow

•     have a ring

•     have a pale cream cap which may have fine scales.

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How to tell the difference

 If the mushroom is young start by inspecting the unbroken ring.   The horse mushroom has a kind of cogwheel ring as shown in this photo. This partial veil is distinctive enough but the inner spokes are fine and  web-like like a cortina.

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Close up showing the ring of Cortinarius australiensis to be a cortina

The picture to the left identifies the ring of the actual poisonous mushroom as a rather thick cortina  that extends over the whole cap  from low on the stem.   However, the cortina  threads  are so  thick that they look more like an agaricus veil.

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If the mushroom is older check the colour of the gills.  The gills of the Horse mushroom change from cream to pink to dark brown with age.

The gills of the poisonous look-alike are  initially very pale mustard and darken to a distinct rust  brown  due to the typical rust coloured spores of Cortinarius mushrooms.   The specimen in this photo shows both a section of newly exposed, pale mustard gills.  Notice in this photo how much the  still attached parts of the cortina looks like  an agaricus ring rather than the usual fine threads a Cortinarius web.  However, the gill colour identifies this mushroom as a Cortinarius.

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Differences in the mature mushroom

The photo on the left below shows a mature Cortinarius australiensis mushroom.   Here, the gill colour could be mistaken for those of the horse mushroom pictured on the right.  However, the Cortinarius gills are more rusty  than the Agaricus gills and the real give-away is the distinctly rust coloured spore stain  on the stem of the Cortinarius. Notice also the chiselled shape of the  Cortinarius base.  

The gills of the horse mushroom are chocolate brown even at this fairly early stage.   Notice also how much longer the stem is in the horse mushroom.

Side view of a mature Cortinarius australiensis mushroom showing rusty spore stains on the stem.

A maturing horse mushroom showing chocolate brown gills.

In conclusion, if this comparison makes you rather more wary  about any "horse mushroom" you might throw in the  pan, it is probably a good thing. These two species can look very alike.