Conidial and microconidial ontogeny of Marssonina brunnea, M. castagnei, and M. populi was annellidic. The primary conidium and microconidium developed holoblastically and a new slightly higher conidiogenous locus was formed with each percurrent vegetative proliferation through the annellophore apex. Delimiting septa were perforate with Woronin bodies and electron-dense septal pore plugs were observed in the bases of seceded conidia and microconidia.