AbstractTests conducted to evaluate the efficacy of C. tenuissimum, a destructive hyperparasite of rust spores, against rusts in the genera Melampsora, Cronartium, Peridermium, Uromyces and Puccinia are presented in this chapter, with emphasis on its modes of action, the fine-level analysis of the fungal host-hyperparasite interface, the antifungal compounds it produces, and its ability to reduce disease both in vitro and in planta. The characteristics of the fungus that enable it to survive in natural habitats and retain activity under varying environmental conditions are described.