The winter mushroom, Flammulina velutipes, is one of the major economical crops cultivated in Korea. The total production have steadily increased approximately 40,161 M/T in 2005 to 61,057 M/T in 2009. Several bacteria have been known as the causal agents of certain diseases of cultivated button mushroom(Agaricus bisporus) oyster mushroom(Pleurotus ostreatus) and winter mushroom(Flammulina velutipes). It is well known as bacterial diseases of the cultivated mushroom such as brown blotch, mummy disease, bacterial pit, bacterial rot, weeping disease, ginger blotch, and drippy gill. Black rot has been recognized as a major problem within the mushroom industry. Pseudomonas tolaasii has been shown to be associated with a black disorder of the caps and stipe of the mushroom. Recently, P. tolaasii was isolated from disease cultivated winter mushrooms grown in Korea. Its symptom appeared as dark brown and sunken lesions on the caps and stipes of affected mushrooms. Inoculation of bacterial isolates into mushroom caps and stipes showed characteristic black rot symptoms and sunken lesions. Results of Gram stain, staining of flagella and biochemical tests identified these isolates as P. tolaasii. This was confirmed by pathogenicity, physiological and biochemical characteristics, and results of an analysis of the 16S rRNA gene sequences.