Fairy rings are caused by a fungus that often shows up in locations where trees once grew or in lawns with a great deal of thatch. Typical symptoms are a ring or arc of darker-colored grass with mushrooms growing in the darkened area. Sometimes a dead patch accompanies the ring. They generally appear in spring and go away quickly. They’re almost impossible to eliminate, but you can mask their presence. If the circle within the ring is more yellow than the rest of the lawn, fertilize the entire area. That will help even out the coloration and make the fairy ring less noticeable. Extra irrigation also sometimes helps, because the soil-inhabiting fungi create artificial drought conditions by sucking up extra soil moisture.