It would start with the rye becoming infected during the summer and eventually leading into the winter of 1691. These girls that started proclaiming that they saw neighbors with the Devil could actually be experiencing hallucinations of this. The girls and Tituba could actually be experiencing these things not because of witchcraft but because they hallucinating these events. Therefore if there was an outbreak of ergot these girls really wouldn't be lying they would just be telling what they see.
If an ergot outbreak was present it would also explain the the other symptoms displayed by the sick girls such as Betty. Along with hallucinations ergot muscle spasms and a coma like state that would clearly go along with what Betty was displaying while sick. These common symptoms also displayed by others in the book do point in all directions of an ergot infection. Ergot which thrives in warm and damp conditions would surely grow in Salem during the spring and