The significant issue in using fungicides for lawn diseases is that by the time the disease is recognized and the specific fungal cause identified, the infection has usually run its course. The damage is done and no amount of fungicide can restore the damaged grass. At best, spraying the healthy grass in the area with a fungicide may retard disease spread. The lawn care measures you adopt (topdressing, watering and fertilizing properly, mowing at the correct height, aerating, and reducing thatch) more effectively address present and future disease problems in most lawn disease situations.
A single fungicide will not control all fungal diseases. Fortunately, your lawn is not likely to experience a lot of different fungal diseases. Have a sulfur-based general garden fungicide on the shelf, because it does control several common diseases, including dollar spot, brown patch, snow mold, leaf spot, powdery mildew, and rusts. Sulfur has been used against plant pests and diseases for thousands of years. It is available for use in an appropriate pesticide applicator as a fine yellow wettable powder (formulated to be mixed with water), or as a dust or a liquid concentrate. Do not use it when temperatures are higher than 80°F to avoid toxicity damage. Whatever product you use, always follow the label instructions carefully.
For information on fungicides available in most garden centers go to Disease Control Products