菇 is a bound form — it almost never stands alone. It appears as a suffix in mushroom names:
蘑菇 mógu (button mushroom, the generic term),
香菇 xiānggū (shiitake),
金针菇 jīnzhēngū (enoki),
草菇 cǎogū (straw mushroom),
杏鲍菇 xìngbàogū (king oyster). For 'a mushroom' on its own, use
蘑菇.
