Why? Because of its high susceptibility to infection by a fungus by the name of Claviceps purpurea, otherwise known as ergot. It might not sound terribly dangerous, but trust me, ergot is no ordinary fungus. It does a whole lot more than cause unsightly yellow toenails. Ergot is thought to be have caused the deaths of thousands — maybe hundreds of thousands — of people over the course of Western history, caused the fall of kingdoms and laid waste to entire societies. No small feat for a spore.
But what is ergot exactly? Well until about 1850 it was so common in rye that every previous rye cultivator simply thought it was a natural part of the plant, not the infectious, toxin-producing life form that it really is. Ergot enters the rye plant through the flower in the springtime, especially in very wet years. As it grows, it replaces some of the grains on the ends of the ears producing long, dark brown growths known as “cockspurs”. It is these growths (technically known as sclerotia) that are the source of the problem in humans. However since no one understood that until the industrial age, they were simply treated like a normal aspect of rye grain, and ground up with the rest of the healthy rye to make flour.
The problem is that ergot produces a variety of toxic alkaloids, one of which is lysergic acid, a chemical cousin of lysergic acid diethylamide, better known as LSD.