Dr. Garrett mentions a theory that the incidents that sparked the Salem witch trials were triggered by hallucinations caused by eating ergot contaminated by the the mold c. purpurea (a type of ergot). However, this theory was not developed until 1976. (The numerous effects of ergot were well-known as St. Anthony's Fire, but nobody had theorized that it caused the hallucinations that were behind the accusations of witchcraft in Salem.)
Luke tells Cora that "we now believe" that the Salem witch hysteria was caused by the girls eating poisonous mold found on grains. This theory, that ergot poisoning caused the bizarre actions of the girls, was only formulated in 1976, when scientist Linnda Caporeal published a paper making the case for it.