MORELIA, Mexico—There’s an almost Mediterranean charm to the rolling hills here in Michoacán, a state in western Mexico. Avocado farms occupy vast stretches of land, and the rows of low-growing trees resemble the olive gardens of southern Europe. These idyllic farms grow millions of pounds of avocados that Americans consume every year. But there’s a dark story lurking beneath the surface of the fleshy green fruit—and the bowls of guacamole it produces.