This spring, I’ve noticed this disease on about 9 of 10 lawns I’ve looked at.  Most if the sites have been on side yards where there is little air movement and poor light.  Powdery mildew appears on the upper portion of the grass blade as a powdery white substance.  This fungi is usually purely aesthetically displeasing, but can cause the plant to die if it is allowed to become severe enough to infect the crown portion of the plant.  The best way to control the disease is to allow light penetration and better air movement.  There are a # of fungicides labeled for control and suppression.