The quantitative growth of coffee production and consumption in the nineteenth and twentieth centuries produced qualitative transformations along every step of the coffee commodity chain. The economic integration of the global coffee market in this period triggered major east-west biological exchanges between the world's coffee regions. The global epidemic of coffee leaf rust, caused by the fungus Hemileia vastatrix, illustrates the ecological and economic impact of such exchanges. Between 1865 and 1985, the epidemic spread from its original focus in Ceylon to engulf all of the world's coffee zones. Its economic impact varied considerably: in some places it destroyed more than 90% of the coffee crop, while in others it was little more than a minor irritant. The epidemic's origins, its diffusion, and its impacts were not accidental, but reflected specific conjunctures of local and global biological and historical processes.