The black rot pathogen of Pachyphytum ‘Dr Cornelius’ in Suqian City, Jiangsu Province, was isolated by tissue isolation and its taxonomic status was identified based on morphological characters, 16S rDNA-ITS sequence analysis and pathogenicity tests. In additional, the effects of temperature, light, pH, carbon sources and nitrogen sources on mycelial growth and conidial production of the pathogen were determined. The results showed that the black rot pathogen on Pachyphytum ‘Dr Cornelius’ was identified as Colletotrichum siamense. The optimum temperature for both mycelia growth and sporulation was 30 ℃, the hyphae grew extremely slowly and stopped sporulation at 5 ℃, the hyphae died and stopped sporulation at 40 ℃; the optimal pH for mycelia growth and sporulation was 5.0 and 7.0, respectively; the pathogen could use a variety of carbon and nitrogen sources, maltose was the best carbon source for mycelial growth and sporulation, peptone was the best nitrogen sources for sporulation, yeast extract was the best nitrogen sources for mycelial growth, but ammonium sulfate inhibited mycelium growth and sporulation. In addition, alternation of light and dark was suitable for mycelial growth and continuous darkness was suitable for sporulation.