Introduction to orchid rust and its pathogen Uredo oncidii

Synonym: Hemileia oncidii Spermogonia (O) and aecia (I) are unknown; uredinia (II) form on the underside of leaves, producing exposed, spreading patches of deep orange powdery urediniospores; telia (III) have not been described. (1) Damage: this rust was first found in France, and was subsequently found in Brazil, South America and the United States. Its host plants include: Epidendrum vitellinum, Lycaste skinneri, Odontoglossum histoniensis, Oncidium bicolor, O. cavendishianum, O. cephalotes, O. forbesii, O. lanceanum, O. pusillum, O. splendidum, O. varicosum. (2) Symptoms: similar to many other rusts, the spore clusters are produced in nearly circular lesions on the underside of the leaves, containing orange-yellow urediniospores; as time passes, the lesions turn brown and necrotic, after which the central part of the lesion dies and turns brown, but as it continues to develop, its margin remains active and orange-yellow. (3) Control methods: all diseased plants should be destroyed, and allorchids, especially susceptible species, should be sprayed with ferbam, and constantly observed for the occurrence of rust. Liu Zhongjian