I remember a book dealing with a valley community that had psychokinetic hive mind powers caused by some fungus in a cold storage cave. I remember the town being a bit isolated and that they bought their gas for the gas station annually. An outsider trying to find out about it nearly gets killed when a pickup truck he bought catches fire/ collapses on a lift. The book is fairly old.
This is Frank Herbert's The Santaroga Barrier published in 1968.
Wikipedia describes how the the main character Dasein infiltrates the town of Santaroga...
...and quickly learns of 'Jaspers', an additive in the food and drink commonly ingested in Santaroga that seems to imbue the consumer with greater health and an expanded mind. Within the Santarogan community Jaspers was described as "Consciousness Fuel" which opened a persons eyes and ears, and turned on their minds. Those who consume it don't become psychic; instead, they're simply far more lucid than the average citizen of the U.S, although there are numerous hints at a group mind operating at a subconscious level.