Abstract: In order to analyze the function of PacC, a pH-signaling transcription factor of Penicillium italicum, which was a postharvest pathogen of citrus fruits. The gene had 1921 bp cDNA and an intron, which encoded 636 amino acids and contains three typical zinc finger domains of transcription factors. Phylogenetic tree analysis showed that the PacC was clustered with Penicillium digitatum and Penicillium chrysogenum. In vitro test, the PacC gene was stably expressed during the growth of P.italicum. The pH of the medium affected the growth of P.italicum and the expression level of PacC, and the expression level was significantly down-regulated under acidic conditions(P<0.05), but significantly up-regulated under alkaline conditions(P<0.05). Carbon source conditions affected the expression of PacC. The expression of PacC was significantly increased by glucose starvation and supplementation(P<0.05). The pH alkalization of the medium was caused by low concentration sucrose, which significantly stimulated the expression of PacC(P<0.05), while the pH acidification of the medium was caused by high concentration sucrose, which gradually decreased the expression of PacC(P<0.05). In vivo test, the inoculation of different citrus varieties with P.italicum would lead to the decrease of citrus peel, and the initial pH of inoculation had an important effect on the pathogenicity of P.italicum. The PacC gene was stably expressed throughout the infection process of P.italicum on citrus. These results indicated that both environmental pH and carbon source could affect the expression of PacC and the pathogenicity of P.italicum.