Microfungi growing on plants were studied in protected and disturbed oak forests. Natural and planted oak stands are the dominant forest ecosystems in Ukraine. Fungi, including microfungi, are diverse in these ecosystems: 83 species of ascomycetes and 73 species of conidial fungi are known from oak forests, woods, stands and shelter belts. The influence of air and other types of human pollution on diversity of these fungi was estimated. Certain wood-inhabiting ascomycetes (e.g. Diatrypella quercina, Pseudovalsa umbonata, Diaporthe leiphaemia) are known to contribute to oak branch dieback and bark lesions, especially in trees weakened by other factors. These fungi seemed more widespread and abundant in disturbed oak forests. Comparison of fungal diversity in various oak-forests showed that in deep-crisis ecosystems the number and distribution of leaf-inhabiting conidial fungi declined, and this decrease was correlated with the human disturbance. The common group of leaf-inhabiting fungi (Ascochyta quercus, Discula quercina and Septoria quercicola) was not observed in polluted or disturbed oak forests. Change in status of such groups and reduction of dominant conidial fungi may be potential indicators of the state of plant communities in disturbed ecosystems. The study was supported by the Ukrainian State Fund of Fundamental Research.