Orchid rust is caused by the rust fungus Pucciniastrum philippinense

(Uredo behnickiana), synonym: Uredo americana. The spermogonial stage (O) and aecial stage (I) of this pathogen are unknown. The uredinia (II) mostly form on the underside of leaves and produce orange-yellow urediniospores; telia (III) have not been observed. (1) Damage: This pathogen was first discovered in 1905 on Oncidium in Germany, and this rust disease was first discovered in 1954 on Phaius in Cuba. The disease weakens plant growth and, in severe cases, kills the plant. The species affected include Cattleya citrina, C. dowiana, C. dowiana aurea, C. guttata, Epidendrum vitellinum, x. martiae, E. alatum, Laelia anceps, L. autumnalis, Oncidium cavendishianum, O. crispum, O. marshallianum, O. varicosum, Schomburgkia undulata, Phaius grandifolius, and others. (2) Symptoms: On Phaius, the disease appears as patches of orange-yellow blister-like pustules on the underside of leaves. The patches are small at first, then rapidly expand into circular rings, until finally the entire underside of the leaf is covered with pustules. On the upper surface of the leaf, the areas corresponding to the patches show yellow-green chlorotic zones. Severely infected plants can still flower, but compared with normal plants the flowers are fewer and smaller, the leaf area of each successive flush of new leaves gradually shrinks, and eventually the plant becomes fully chlorotic and stunted in growth; such plants should usually be discarded. (3) Control methods: Once the disease has developed, it cannot be controlled. The pathogen spreads extensively within the leaf, and its hyphae can extend within the leaf far beyond the area where spore pustules are produced beneath the epidermis. The pathogen cannot enter the growing point and is confined to the leaves. There is no feasible chemical treatment, so the only option for this disease is to destroy all infected plants, and for highly susceptible varieties, inspections must be carried out weekly without fail. Liu Zhongjian