Ring rot induced by Botryosphaeria dothidea is a major cause of growth and postharvest losses in various fruits. There is an urgent need to develop green fungicides due to pesticide resistance and environmental pressure. Here, we demonstrated the efficacy of dictamnine (DIC, 4-methoxyfuro [2,3-& beta;] quinoline, purity 98%), a compound isolated from the stems and leaves of Clausena lansium, in effectively suppressing pear ring rot by inhibiting the mycelial growth of B. dothidea. The median effective concentration of DIC was 15.48 & mu;g/mL. Application of DIC to B. dothidea resulted in structural disruption of the cell wall and plasma membrane, leading to mycelial deformation, breakage, and cell death. Transcriptome analysis revealed significant inhibition of the synthetic pathways for fungal cell wall and membrane components by DIC. Particularly, the expression of chitin synthase, a key enzyme of chitin synthesis, was prominently down-regulated. Moreover, the chitin content in DIC-treated B. dothidea mycelia exhibited a substantial dose-dependent reduction. Based on these results, it is promising to develop DIC as an antifungal pesticide for controlling ring rot disease in pear fruits. Our study provides new insights into the underlying mechanism through which DIC inhibits the mycelial growth of B. dothidea.