Conidial state of Blumeria graminis on Kentucky Blue Grass
Micrograph from MyoAlbum CD
It is the long chains of spores over the surface of the infected leaf that gives the powdery appearance to the symptoms. The spores look blue here due to phase contrast illumination but normally spores are colourless and the affected areas on the leaf appear chalk white by reflected light. The green in the bottom left of the photo is the chloroplasts.
One of the remarkable features of this disease is that the fungal hyphae do not penetrate the host leaf. The only part of the parasite that penetrates into the plant are specialized feeding cells or haustoria (sing = haustorium) that looks like tiny octopus-like things (CLICK HERE).