Powdery Mildew is a fungal disease that can affect lawns in both wet and dry conditions but is most prevalent in cool, dry weather and high humidity. In fact, powdery mildew spores require dry conditions to spread and are suppressed by excess moisture. It is identifiable by the white to light gray “powder” of spores and fungal strands that form on grass. It is especially common on lawns grown in too much shade.
Water properly, wetting the entire root zone (6-8 inches deep) and allowing the turf to partially dry between irrigations (web link?). Decrease shade (prune trees) or switch to more shade tolerant turf grass. Mow at the proper height and avoid over-fertilizing.