SF: Death of Grass by John Christopher

Ok, opposite end of the spectrum from Mockingbird this time. The English go brutal, more than a hint of J.G. Ballard there then. A virus breaks out in the far east which attacks some kinds of grass. Gosh, isn't that a thing? Then it diversifies and starts to attack all grasses: rice, wheat, oats, you name it. Suddenly it becomes rather more significant doesn't it old chap? Better plant the potatoes then.

Much of the story takes place over a couple of days post-tipping point, less "old chap", more shoot first and kill the bast**d. Not one to read if you're of a sensitive disposition, but very gripping.