Abstract: Broccoli brown stem, a new disease, was widespread in Zhejiang Province in recent years. To understand and control the disease, lots of model symptom samples were collected to separate and culture. The results indicated that the pathogen, as an obligate parasite, depended on the host plants and could not be cultured in vitro. In microscope, the sporangiophores of pathogen tapered on the tips with a shape like letter “Y”, the sporangiums of which were oval or eggs, (24-27) μｍ×(15-20) μｍ, then the oospores with thick wall were roundness with snuff color. When inoculated some mycelium from sick tissues on the surface, the symptoms same as in fields appeared on healthy broccoli leaves and stems. On the other hand, rDNA ITS (Internal Transcribed Spacers) of pathogens were cloned and sequenced, and its similarity to Hyaloperonospora parasitica was more than 95%. Combining the molecular results with the morphological characters, it can be definitely certain that broccoli brown stem was caused by Hyaloperonospora parasitic.

Key words: Brassica oleracea, brown stem disease, Hyaloperonospora parasitica