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Common hazel (Corylus avellana) is an economically valuable nut crop worldwide. Phyllactinia guttata is a widespread powdery mildew species on this host in many hazelnut‐producing countries including Iran. Here, we report a new powdery mildew species on C. avellana in Iran. Hazelnut trees showing severe powdery mildew infection symptoms on leaves and fruit clusters were collected from forest areas in the Ahar (East Azerbaijan province) and the Khodaafarin (Ardabil province) regions of Iran. Based on a polyphasic taxonomic approach including morphological features of asexual and sexual states and phylogenetic inference based on ITS‐rDNA sequences, the identity of the species was determined as Erysiphe corylacearum. This study provides the first report on the occurrence of E. corylacearum on common hazel in Iran. [ABSTRACT FROM AUTHOR]