The fungus theory is a valid one. Ergot (the fungus) is the base component in LSD.
You’ve already answered your own question. Part of what made the Salem Trails a tragedy, a horror, was ignorance. They had no clue that a fungal infestation of their Rye crop could possibly cause mass hallucinations. Add that to a puritanical religious community, a hard scrabble exsistence? All of these variables were the pressures of that particular society. We know the outcome, but can only guess at the absolute cause and effect.
Fear of the unknown will always be the most powerful tool.
Research subjects that might seem mundane. The Green Mile by Stephen King uses the history of the electric chair as a fear selling point. In reality one of the very first executions turned out almost exactly as portrayed in the book/movie. History when you delve into it has had many weird, horrifying incidences. Truth is always stranger than fiction.