In the southern corner of Guatemala, outside the tiny mountain town of San Pedro Yepocapa, Elmer Gabriel’s coffee plants ought to be leafed-out and gleaming. It is a week before Christmas, the heart of the coffee-harvesting season, and if his bushes were healthy, they would look like holiday trees hung with ornaments, studded with bright-red coffee cherries. But in a long row that stretches down the side of his steeply sloped field, the plants are twiggy and withered. Most of their leaves are gone, and the ones that remain are drab olive and curling at the edges. There are yellow spots, brown in the center, on the leaves’ upper surfaces. On the underside they are pebbly, and coated with a fine orange dust.