Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales, with Podosphaera xanthii (a.k.a. Sphaerotheca fuliginea) being the most commonly reported cause.Erysiphe cichoracearum was formerly reported to be the primary causal organism throughout most of the world.owdery mildew is one of the easier plant diseases to identify, as its symptoms are quite distinctive.