An overly watered or overly fertilized lawn is prone to mildew, which can coat your grass with a white sheen. Lay off the fertilizers, and water in the morning rather than early evening. Leaving grass wet overnight makes it more prone to mildew and other fungal diseases. Also, water infrequently and deeply, which not only cuts down on mildew problems, but it also allows your grass to grow a deeper root system that makes it less prone to weeds. Grass that gets a lot of shade can be prone to mildew. If you see it in a shady area, consider replacing the grass that’s there with a more shade-tolerant variety.