Well, their genetic uniformity could be the basis of vulnerability to devastating epidemics. It takes 10 to 20 years to develop resistant strains against fungi, aphids, insects, and diseases. When all the plants in a single field are near clones of one another all it takes is one outbreak of insects or disease to destroy all the plants of the entire monoculture. The Irish potato blight is an example we all know. Presently agroscientists are trying to stay ahead of the Russian wheat aphid, which arrived in Colorado in 1987.
Wheat is Colorado's leading cash crop.
Spraying herbicides is only a temporary solution due to the expenses and ability for the aphids to develop their own genetic resistance to the herbicides. Half of the Colorado acreage of wheat is of only one variety, TAM107. The U.S. is heavily dependant upon only four inbred lines of corn. Due to the influence of the market that sells McDonalds Russet Burbank potatoes for their fries, Burbank potatoes are becoming the dominant potato planted in Washington State. We may have to change our preference for Russet Burbank potatoes.