Are My Michelin Tires Dry-Rotting?

I had the same experience with the Michelins on my 2011 Chevy Dually. The 6 tires on the vehicle had very minor checking, but the spare was bad. The local Michelin dealer had a template with pictures of various degrees of checking. Mine matched the one which said to replace the tire. They did so for free without question. The explanation for my spare being worse is that there are chemicals in the rubber which when "worked" by usage stretch the rubber and keep the anti-aging chemicals active. My spare left sitting motionless was worse than the other 6.