SUMMARYThe developmental cycle of a fungus commonly known asPiggotia Fraxinihas been studied. The fungus attacks various species of ash throughout the United States and is especially common in forest-tree nurseries.The pathogen is polymorphic, possessing a conidial stage, properly identified asMarssonia FraxiniEllis&Davis, a spermogonial and carpogonial stage, commonly designatedPiggotia FraxiniBerk.&Curt., and a perithecial stage, herein assigned toMycosphaerella effigurata(Schw.) House. Evidence of a genetic connection of these stages is presented and an emended description of the fungus is included.The pathogen is commonly associated in nature with such other species asMycosphaerella fraxinicola(Schw.) House,Sphaerella fraxineaPeck,Cylindrosporium Fraxini(Ellis&Kellerm.) Ellis&Ev.,Gloeosporium punctiformeEllis&Ev., andCylindrosporium viridisEllis&Ev.