Which is an edible Fungus?

Concept — Fungi are heterotrophic eukaryotes with chitinous cell walls, and they are separated first by how they obtain their nutrition and then by the kind of reproductive body they build. In classification, the term “edible fungus” names the fungi that are harvested and eaten as a food in their own right — that is, the mushrooms. Two things must hold together for a species to qualify. First, it must build a fleshy, macroscopic fruiting body that can be gathered and cooked: the basidiocarp of a basidiomycete such as Agaricus or Volvariella, or the ascocarp of an ascomycete such as Morchella, the morel. Second, that particular species must be established by long use to be non-toxic, because the fleshy body by itself proves nothing — many mushrooms are inedible and some are deadly. Micro-fungi used industrially as fermenting agents, such as brewer’s yeast or the moulds of certain fermented foods, are not what this term picks out. Fungi that build no macroscopic fruiting body at all fail the first condition outright: the obligate parasitic rusts, and the microscopic moulds whose spores are formed in tiny pustules or inside sporangia.

Application — Among the four genera offered here, only Agaricus is a saprophytic basidiomycete that produces a fleshy basidiocarp, and its species are long established as human food. Agaricus bisporus, the button mushroom, and Agaricus campestris, the field mushroom, are cultivated on composted manure and eaten as a vegetable across the world. Agaricus is therefore the edible fungus here.

Cross-check — The other three genera are ruled out on the same two criteria, mode of nutrition and type of fruiting body:

• Puccinia is an obligate plant parasite, a rust fungus. Puccinia graminis tritici causes black stem rust of wheat and completes its life cycle on wheat and barberry, producing rust-coloured spore pustules on leaves and stems; it forms no fleshy fruiting body.

• Mucor is a zygomycete pin mould with a coenocytic, non-septate mycelium. It grows on stale bread, dung and decaying organic matter, and its spores develop inside a pin-head-sized sporangium; it is a spoilage organism.

• Rhizopus is the zygomycete bread mould. It spreads over the substratum by arching stolons anchored by tufts of rhizoids and causes soft rot of bread and stored fruit; its sporangia too are microscopic.

Result — Hence the edible fungus among the four is Agaricus, the mushroom.