When disease or insect damage appears on lawn, turf managers on golf courses, in parks and on sod farms don't reach for fungicides and insecticides. Those products are expensive and often deliver uncertain results. Instead, a good greenskeeper aims first at eliminating the underlying "strikes" against the grass plants, to make the lawn stronger.
Fungi and insects can cause trouble in a lawn but unless the lawn's weak for other reasons very few of them can become major problems. Blade by blade, grass that's strong can resist infection, slow the spread of a pest, and grow leaf surface faster than a pest can destroy it.
So if you see lawn problem, strengthen the grass plants. Start in spring with aeration, steady watering and fertilizer to shore up their overall vigor. That way, even if a plant loses some foliage each year to the likes of mildew or leaf spot when wet, cloudy, cool conditions favor fungus growth, it will be able to fill back in during drier, warmer, sunnier, periods. (Probably you've seen tall phlox plants do this after a mildew infection, and scab-infected crabapples, too. The same plants that were white-coated or leafless in late fall come back fresh and lovely in spring.)
In addition, any time you see disease, overseed with premium grass seed to introduce fungus- resistant grass varieties. They will fill in where the current crop hasn't coped.
Shaded grass is weaker than lawn in sunny spots. So if the problem area is in the shade, choose shade-tolerant seed.
It's a great move for grass growing on compacted soil.
Afterward, prevent compaction's return. This might mean establishing paths if foot traffic is the problem. Cover a path with mulch to cushion each footfall or stepping stones to spread a body's weight like a snowshoe over the soil. As an alternative, install and plant a paving grid.
Finally, considering switching from conventional high nitrogen granular fertilizers to slow release organic products. Some diseases, such as powdery mildew of grass, are encouraged by heavy nitrogen. Slow release organic products don't invite fungi and do improve soil.