The scene couldn’t be more British. There are picnic blankets, dogs and hungover-looking men shouting at their children. Thanks to recent rain, mushrooms shaped like huge white breasts have sprouted from the tussocks. Naturally, everyone ignores them, except for an inquisitive toddler, who reaches out a pudgy hand, before it is quickly snatched away. “Don’t touch,” an adult hisses. “Those things kill.” In the UK, mushrooms are the vegetable we love to hate. But in recent years, they’ve been hard to avoid. They’ve sprouted from our bookshelves (Merlin Sheldrake’s bestselling Entangled Life) and have popped from our screens (Fantastic Fungi;...