Powdery mildew diseases previously unrecorded in Korea were noticed on four species of ornamental trees. Field observations on each disease were described and the causal fungi were identified. Powdery mildew of Symplocos chinensis for. pilosa was found to occur mildly, especially on plants growing in shade. The causal fungus was identified as Microsphaera nomurae U. Braun. Cornus florida was found to be infected with Microsphaera pulchra Cooke & Peck and supposed to be epidemic in nursery. Powdery mildew of Malus baccata occurred on young leaves and herbaceous stems in nursery. The causal fungus was classified as Podosphaera leucotricha (Ellis & Everh.) Salmon. Powdery mildew of Ulmus macrocarpa was found to occur in autumn and severe on young shoots. Uncinula kenjiana Homma was identified as the causal fungus.