Sporulation, septal pore apparatus and host-parasite interaction between Us
tilago osmundae and its host, the royal fern, were analyzed by light and el
ectron microscopy using herbarium material and compared with findings in ot
her species of Basidiomycota. The fungus produced thick-walled and thin-wal
led hyphae on leaf surfaces and sporulated externally. Coralloid haustoria
with numerous lobes were present only in epidermal cells. Hyphae in interce
llular spaces were lacking. The septal pore apparatus was composed of a sim
ple pore with two caps. Ustilago osmundae shares the type of septal pore ap
paratus and host-parasite interaction with the Melanotaeniaceae of Ustilagi
nomycetes. It differs from the other species of this family primarily in th
e external sporulation and in the lack of intercellular hyphae. To accomoda
te this unusual fungus in Melanotaeniaceae a new genus, Exoteliospora, is p
roposed. Description and illustrations are given for Exoteliospora osmundae
.