Golovinomyces orontii is a powdery mildew fungus in the family Erysiphaceae, an obligate biotrophic pathogen causing white powdery coatings on a broad range of host plants including Arabidopsis thaliana. It has become an important model pathogen for studying plant-fungal interactions and immune responses due to the widespread use of Arabidopsis as a model organism. Spores disperse by wind and the fungus completes its lifecycle entirely on the surface of living leaf tissue.