Powdery Mildew Is A White Fungus
This particular lawn disease is largely cosmetic and doesn't usually cause damage to the affected grass, although it can become distracting and unsightly in your yard.
Appearance
The areas of affected grass start out in small patches, but can spread to affect larger areas, since the spores spread on the wind. It will generally only take hold in the areas that provide the best conditions to powdery mildew development: damp conditions in shady areas.