Answer: What you describe sounds like a fungal disease. There are several grass fungus types:
Fairy Ring. This type of grass fungus leaves unsightly rings of dark green and dead grass, with brown mushrooms in some extreme cases. The fungus feeds on dead vegetation, including stumps and old thatch, beneath the surface of your lawn. Spike the rings so water can penetrate.
Pink Snow Mold. Irregular and brown patches of brown and yellow grass, with white or pink cottony fungus after a heavy rain. This is the result of a heavy snow cover, such as hills or banks. Keep the grass mowed and avoid nitrogen application in late fall.
Rust. This grass fungus turns your lawn light green or yellow, with yellow to brown spots in late summer. Heavy dew favors this disease and activates in warm or moist weather. Maintain vigorous growth with water and a nitrogen fertilizer.
Slime Mold. This is a white to yellow to grey slime that clings to beads like leaves. Occurring during prolonged wet periods, this grass fungus is helped by raking or mowing.
Once you determine which is the problem you can take corrective action including using fungicides if necessary. Best wishes with your lawn!
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