Annotation: Powdery mildew Erysiphe alphitoides is one of the most important pathogenic fungus infecting pedunculate oak Quercus robur in Europe. As the identification of this species with morphologic markers is unreliable, the molecular markers development is necessary. In this work, we developed primers for PCR amplification of cytochrome b gene fragment. The results show high level of intraspecific variability of this mitochondrial gene and revealed the indistinct taxonomy relations within E. alphitoides s. lat group.