Curvularia lunata (Wakkre) Boedijn is an important pathogenic fungus that causes maize leaf spot, a prevalent disease that caused significant yield losses in maize-growing areas in China in the 1990s. Clpks18, a polyketide synthase (CLPKS18) gene, has been identified as a crucial virulence-related gene in C. lunata. However, the impact of Clpks18 and its biosynthesized virulence factor (R)-(-)-mellein on the expression of maize genes related to the defense signal pathway has never been determined. In this study, it was found that Clpks18 and (R)-(-)-mellein significantly interfere with the signaling pathways of JA and IAA in maize leaves but in different ways and in a time-dependent manner. While CLPKS18 inhibited the maize’s JA and IAA signaling pathways through its related secondary metabolite, (R)-(-)-mellein inhibited the JA signaling pathway but stimulated IAA accumulation in maize leaves. In summary, understanding this novel virulence effector’s mechanism of interference with maize resistance enriches the pathology of Curvularia leaf spot in maize on the one hand and provides a foundation for screening the resistance germplasm and chemical fungicides against the disease on the other.