The effect of oak powdery mildew (Erysiphe alphitoides (Griffon & Maubl.) U. Braun & S. Takam.) on the most important plant physiological processes, photosynthesis, transpiration, and stomatal conductance, was studied on pedunculate oak saplings, depending on leaf age and fungal infestation intensity. The intensity of net photosynthesis declined progressively with mycelial development on the leaf surface, regardless of leaf age. The reaction to the presence of pathogenic fungi was more intensive on the young leaves. Net photosynthesis was practically interrupted (0.02 μmol m-2 s-1) under the highest degree of young leaf infection (> 75%). Powdery mildew exerted a moderate effect on transpiration rate, and a decreasing trend was observed with an increase in leaf infection. The effect of powdery mildew on the leaf stomatal conductance was not significant, and the values were, on average, higher in older leaves. The concentration of photosynthetic pigments decreased linearly with an increase in leaf infection regardless of the age. At the same time, the leaf dry matter content increased with higher leaf infection and, on average, it was higher in older leaves. The results of the study point to the adverse effect of powdery mildew on net photosynthesis and other physiological processes in pedunculate oak leaves, allowing them to be used as an important indicator of plant vitality and vulnerability.

Key words: oak powdery mildew, pedunculate oak saplings, net photosynthesis, transpiration, stomatal conductance.