Ozone has been proposed as a convenient elicitor against pathogens since it is known to generate different reactive oxygen species (ROS) and induce nonspecific defence by altering gene expression. The mode of action and its interaction with other defence pathways are yet to be elucidated. Besides its negative effects on plants, ozone can be used for triggering defence against environmental stresses, including pathogens, when used at appropriate concentrations. Powdery mildew, caused by the obligate biotrophic fungus Blumera graminis f.sp. tritici (Bgt), is an important plant disease that reduces crop yield and quality. We hypothesised that ozone treatment may elicit defence against Bgt by inducing ROS signalling or other routes such as the salicylic acid (SA) or jasmonic acid (JA) pathways. We conducted experiments with Bgt-susceptible (cv. Pamukova) and resistant (cv. Tahirova) wheat (Triticum aestivum L,) cultivars and treated them with different ozone concentrations before Bgt inoculation. Stress response and defence-related features such as antioxidative enzyme activity; lipid peroxidation; H2O2 and Ca+2 levels; PR1, LOX, PAL and RBOH gene expression; and disease severity were assayed. Clear discrepancies between the responses of susceptible and resistant cultivars were found, suggesting that different defence routes were activated. Here, we showed that ozone treatment was effective for diminishing Bgt invasion in the susceptible cultivar in the short term, which was probably related to defence induced via the SA pathway. Moreover, the resistant cultivar Tahirova exhibited a different mode of action against the pathogen that was triggered by ozone treatment, plausibly related to the JA pathway.