Oak powdery mildew (Erysiphe alphitoides (Griffon and Maubl.) U. Braun and S. Takam.)) is one of the most common foliar pathogenic organism of oaks, exploiting harmful effects, particularly on young seedlings. To assess and evaluate these negative effects, an experiment was conducted under semi-controlled conditions with 20 one-year-old seedlings of Quercus robur L. infected with E. alphitoides, half of them showing 50% of leaf coverage by epiphytic mycelia and the other half a coverage of 75%. The results of the present study have shown that all observed parameters of chlorophyll a fluorescence transient were highly effective in the detection of severe biotic stress. Moreover, the studied parameters of slow fluorescence kinetics were also considerably influenced, with the coefficient of non-photochemical fluorescence quenching and the Stem-Volmer type non-photochemical fluorescence quenching parameters showing the fastest responses. In case of leaf stomatal traits, the decrease of stomata guard cell width coupled with the increase of stomatal density was observed as the protective mechanism of Q. robur against the stressor. The overall results showed the adverse effects of powdery mildew infection on the photosynthesis of pedunculate oak seedlings, which progressed in time and depended on the severity of the infection. The importance of the results of the present study lays in evaluation and monitoring of the effects of powdery mildew development on the photosynthetic apparatus of one-year-old Q. robur seedlings, which is the most vulnerable stage for the infection by the mentioned pathogen.