Leaf spot and blight disease of tree peony (Paeonia suffruticosa Andr.) was found in an apartment garden in Daegu in May 2003 for the first time in Korea. The causal organism was identified as Phytophthora cactorum (Leb. And Cohn) Schroeter. The causal fungus was homothallic, and produced distinctively papillate, ovoid to subspherical, and caducous sporangia with pedicel. Sporangia that formed in water measured 23.4-42.9×21.5-35.1 μm in range with an average of 35.3±4.6×26.9±3.6.0 μm, l/b ratio=1.31, papillae approximately 3.7 μm high, and pedicels 2.8 μm long. Oogonia were spherical and 21.5-37.1 μm in diameter with an average of 29.6±4.9 μm. Oospores were spherical, mostly plerotic, and light orange brown when mature, and measured 19.5-31.2 μm in diameter with an average of 25.2±4.4 μm. Antheridia were almost ovoid or clubshaped and 11.7-15.6×9.8-11.7 μm in size.