Schiff alludes to it in the foreword. If she discusses it in detail, it’ll be later on. You’re referring to the idea that hallucinatory ergotism contributed to the hysteria. This would’ve been caused by fungus growing on the rye. Ergot can produce a substance similar to LSD, and can produce hallucinations.
Before one dismisses the idea out of hand, an incident of hallucinatory ergotism DID take place in France in the 20th century, in 1951
http://www.thepoisonreview.com/2010/12/05/2114/
…so the idea isn’t far-fetched. And people definitely did report witnessing things that certainly weren’t everyday occurrences, and sure as heck sound like hallucinations.
On the other hand, there were plenty of other pressures on the inhabitants of Salen Village, and it’s not clear that they needed an actual hallucinogen to produce such visions. I feel that ergotism alone also can’t explain everything that took place.
so, at most, it was a contributing factor, but not the decisive one.