powdery mildew

A plant disease, caused by fungi of the order *Erysiphales , in which the pathogen grows as a white powdery coating (of mycelium and conidiophores) on leaves and stems. Examples include powdery mildew of cereals (Erysiphe graminis), gooseberry and blackcurrant mildew (Sphaerotheca morsuvae), and apple mildew (Podosphaera leucotricha). Compare downy mildew.