mushroom has 2 distinct synonyms spread across 1 of its 7 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is toadstool.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
mushrooms and related fleshy fungi (including toadstools, puffballs, morels, coral fungi, etc.)
Synonyms, antonyms and the broader and narrower relations for mushroom come from Princeton University’s WordNet. Full sources and licences.
any of various fleshy fungi of the subdivision Basidiomycota consisting of a cap at the end of a stem arising from an underground mycelium
a large cloud of rubble and dust shaped like a mushroom and rising into the sky after an explosion (especially of a nuclear bomb)
fleshy body of any of numerous edible fungi