It sounds like a few different things got mashed together there. Ergot is a hallucinogenic fungus that grows on rye, and is speculated to be the cause of some of the witch panics. It’s not the same fungus found in Roquefort, but it is what they use to make LSD.

Witches flying is hypothesized to be entheogen use, since a common side effect is feelings of floating, flying, or otherwise ‘being high’.