Corn rapidly depletes the soil of nutrients. Corn's increased use and productivity in this century based on discovery of hybrid corn. More modern concern was how to grow mass quantities of wheat in areas like USA Midwest. Puccinia graminis , named for spore color. Billions of dollars worldwide yearly lost to cereal rusts.
Rusts weaken plants and lower yield.
These are high impact crops that demand a lot of fertilizer, pesticide, water, and are best suited for mechanical farming.