Snow White ate the poisoned (adj.) apple. -> the apple that was poisoned
Snow White ate the poison (n.) apple. -> Snow White ate the apple that was associated with poison.
The English language, at least in my experience, is suffering what I call the "disappearing 'ed' syndrome."Technically only poisoned is right, because poison is a noun or a verb but not an adjective (according to major dictionaries).
But "poison apple" is widely used and sounds idiomatic.
Manchineel - WikipediaWhen ingested, the fruit is reportedly "pleasantly sweet" at first, with a subsequent "strange peppery feeling ..., gradually progress[ing] to a burning, tearing sensation and tightness of the throat". Symptoms continue to worsen until the patient can "barely swallow solid food because of the excruciating pain and the feeling of a huge obstructing pharyngeal lump"