Polyphenol oxidase (PPO) plays a central role in plant defense against fungal pathogens through the oxidation of phenolics into antimicrobial quinones. This study assessed changes in PPO activity in the young shoots and leaves of Rumex maritimus infected with the smut fungus Ustilago parlatoreii across different infection stages. PPO activity was measured at 420 nm for 30–180 seconds under pre-infection, pre-sporulation, young sporulation, mature sporulation, pre-flowering, and flowering stages. In young shoots, PPO activity increased from 0.012–0.044 OD (before infection) to 0.015–0.055 OD during young sporulation, representing the highest enzyme induction. Similarly, leaves exhibited a rise from 0.023–0.031 OD (before infection) to 0.036–0.048 OD during young sporulation, with mature sporulation also showing elevated values (0.030–0.038 OD). Both tissues displayed a slight decline during pre-flowering and flowering, with shoot values decreasing to 0.010–0.023 OD and leaf values to 0.012–0.019 OD, indicating reduced defense activity during reproductive development. The sharp increase in PPO during sporulation underscores its role in oxidative defense and pathogen containment. Overall, the findings confirm that PPO activation in R. maritimus is strongly stage-dependent and peaks during active fungal sporulation, highlighting its importance as a biochemical marker of host response to U. parlatoreii infection.