Your worst-case potato scenario
Exponential population growth
Mass scale monoculture

Between 1700 and 1900, the potato played a major role in Europe's population growth. This crop provided many nutrients and required minimal labour, space and money to grow. Therefore, many farmers relied on potatoes for survival.
The decrease of arable land led to mass-scale cultivation of only this crop. This proved to be fatal once potato blight (mold) ravished crops all around North and Western Europe.
Result? A full-blown food crisis.