There are lots of tremendously useful plants that humanity has domesticated — wheat is useful for bread and beer, cotton and hemp are excellent sources of fiber, cocoa beans have been used as currency, tobacco and coffee beans have opened trade routes on their own, and so on.
But let's say you, the Malicious Omnipotent Deity, decided to remove one plant from the Earth's history. It has to be specifically one — so you could remove Brassica oleracea and nuke cabbage, cauliflower, and brussels sprouts all in one go, but you can't delete all cereal grains.
Which plant would you choose to remove in order to have the most profound impact on human history? Ideally the result will be far-reaching (you could remove tulips to get rid of the Dutch tulip craze, but that doesn't change things much outside of the Netherlands) and significant (apples grow in lots of places, but losing apples doesn't change history much beyond not having apples).