Answer: From your description, it's likely that you have powdery mildew which is a common, but usually not serious problem especially on some varieties of bluegrass. It is accentuated in shaded conditions and with cool, humid or wet weather. Over fertilization can cause rapid spread.
To control, cut back on fertilization and water only in the morning when absolutely necessary. If it occurs in a shady spot, consider pruning back some trees for more light and better air circulation. Although a fungicide could be applied, it will not be effective in controlling the fungus until the cultural conditions have improved.
If reseeding becomes necessary, be sure to get a variety that is resistant to powdery mildew.
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