Abstract: Dendrobium is a famous Chinese herbal medicinal plant and cultivated on a large scale in Simao City, Yunnan Province, China. However, it has suffered from blight in recent years. The aim of present study was to determine the identity of the pathogen causing the disease. Isolates were obtained from diseased plant tissues on Phytophthora selective agar medium and the morphology was studied by growing them on clarified V-8 juice agar medium. The pathogen produced non-caduceus, terminal, papillate and mostly obpyriform sporangia, av. 46.2x34.9 μm with the mean length/breadth ratio as 1.34. Chlamydospores were spherical, thick-walled (av. 30.2 μm diam.). It was heterothallic producing spherical, smooth oogonia (av. 24.0 μm diam.) and short, cylindrical amphigynous antheridia. The cardinal growth temperatures were 9, 27 and 37°C. The Internal Transcribed Spacer 1 (ITS1), 5.8S Ribosomal RNA gene and the Internal Transcribed Spacer 2 (ITS2) and β-tubulin gene sequence of the pathogen were also studied. The results showed 98 and 100% similarity with two isolates of P. nicotianae in the Genbank, confirming the identification as P. nicotianae based on morphological characters. The pathogenicity of the isolates was confirmed by satisfying Koch’s postulates. This is the first world record of Phytophthora nicotianae causing blight of Dendrobium thyrsiflorum, D. chrysanthum, D. aurantiacum and D. chrysotoxum in plantations.