This Lactarius is another agaric of north temperate origin that has made its way into the Neotropics along with its mycorrhizal partner, Quercus. By far, it is the most commonly encountered mushroom in Costa Rican oak forests and seems to produce the most above ground biomass of any agaric we've seen during the course of our project. The large, hard basidiomata with an inrolled flap of sterile tissue at the margin of the pileus, and the copious, white, extremely acrid latex are distinctive.