Ideal fruit is seen as smooth-skinned and bright-hued, a symbol of goodness, kindness, and the virtues of civilization. But not all fruit is easy. In fact, some varieties can be invasive, toxic, and downright difficult. Wheat, for example, is more explosive than gunpowder. Apricot kernels can be deadly to small children. Food writer, poet, and piemaker Kate Lebo shares obscure food history and personal stories in her new book, “Difficult Fruit.”