The Pershing County Sheriff's Office in Nevada tells TMZ ... Tristan Harvey -- the son of Larry Harvey, who founded Burning Man in 1986 and died in April at 70 -- was driving with 2 friends last week into the festival when they were pulled over for not having a rear license plate.

We're told a police dog sniffed the car, honed in, and cops found Tristan was in possession of a mushroom. He was cited for possession of an illegal substance and driving with a suspended license. He was not arrested and told to go on his merry way, sans mushies.

His friend, a female passenger, also had a shroom, and she also was cited and released. Tristan's other friend wasn't so lucky. He was arrested for possession of a controlled substance, but cops aren't saying what they found.

Cops tell us over 70k attended the week-long event. They arrested 44 and cited 53.