No Blade of Grass (1970)

A strange new virus has appeared, which only attacks strains of grasses such as wheat and rice, and the world is descending into famine and chaos. Architect John, along with his family and friends, is making his way from London to his brother's farm in northern England where there will hopefully be food and safety for all of them. Along the way, they encounter hostile soldiers, biker gangs, and all manner of people who are all too willing to take advantage of travelers for a mouthful of food.