You didn't say what kind of grass you have but I think I can safely say that you are taking much too good care of your lawn--lawns that are pampered as your's is often are much more inclined to get diseases. A lot of nitrogen makes for weak foliage growth, tender growth--more liable to get fungal diseases. Are you watering during dark hours? That also encourages fungal disease. If I were you I would cut way back on the fertilizer for a year or so--that alone may help solve your problem.