Powdery mildew fungi are found on many native plants, cultivated crops, ornamentals, and turfgrass species. In general, it is not considered to be a serious disease on turf.
Severe outbreaks of Powdery Mildew on Kentucky bluegrass tend to occur on turf growing in shaded areas during the late spring or mid-fall when moderate temperatures and relative humidity is high.
To help protect you lawn from Powdery Mildew we recommend:
Don't water more than every 3-4 days...only in the morning
Reduce the amount of shade by trimming trees to let more light into the area
Seed with Turf Type Tall Fescue, which can be more resistant to Powdery Mildew than Kentucky Bluegrass
Super Seeding can really help
Core aerate and apply liquid aeration to help water reach the roots
Topdress with compost to help provide soil biology to compete with the fungus that causes Powdery Mildew