A detailed study of a powdery mildew observed on Celtis australis leaves was carried out in the present study. The symptoms appeared as white mycelia on leaves with embedded small black to brown spherical ascomata. Morphological and microscopic analyses of diseased samples revealed that this fungus belongs to Erysiphe section Uncinula. Further investigation identified it as Erysiphe kusanoi which is new to Himachal Pradesh. A description and an illustration of the specimen are given.