Uredo behnickiana (syn. Hemileia americana and H. oncidii) is known on orchids of the genera Cattleya, Epidendrum, Laelia, Lophiaris and Oncidium. Previously, only the uredinial stages of these fungi have been described. The telial stage of the rust species was found for the first time on Oncidium marshallianum. Because of a unique combination of morphological characters the fungus could not be assigned to Hemileia or any other known rust genus. Desmosorus is proposed, therefore, as a new genus with Desmosorus oncidii as the single known species. Desmosorus is characterised by (1) suprastomatal sori on which the spores are formed at the ends of branched hyphae, by (2) subglobose to ovoid, echinulate urediniospores with a smooth patch around the hilum and by (3) thin-walled, oblong elliptical to cylindrical teliospores with a rounded apex.