Characteristics of Photosynthetic Processes in <i>Erysiphe alphitoides</i>‐Infected Leaves of <scp><i>Quercus robur</i></scp> L. Seedlings in Oak Forests of the Steppe Zone, Ukraine

ABSTRACT The vital activity of Erysiphe alphitoides , as a major foliar fungal pathogen, causes the most important biotic stress; being chronic disease, this fungal infection spreads throughout the oakeries in Europe. In some cases, this epiphytotic is considered as the main reason for the slowdown in the natural recovery of the common oak population in Europe. The pathogen has the greatest impact on seedlings, resulting in their mortality. We revealed a clearly confirmed unfavorable impact of powdery mildew on all critical and calculated indicators of the Kautsky curve in the infected Quercus robur leaves. Regardless of the type of oak forest, an influence of the fungal pathogen on photosynthetic processes in annual seedlings has been established. In conditions of E. alphitoides ‐caused biotic stress, Q. robur seedlings experienced violations of the photosynthetic apparatus, progressed over time and depended on the damage severity. The powdery mildew was confirmed to affect most of the plant physiological traits studied. In oak seedlings with the leaf surface damaged by powdery mildew more than 50%, alterations in the chlorophyll fluorescence induction were revealed, quite different in their intensity and direction, compared to the healthy (fungi‐unaffected) leaves. This suggests that all the studied parameters might be used as indicators of the most stressed state of common oak seedlings.