http://www.mg.umn.edu/FAQ/turf.htm I had to look this one up Man.
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Mushrooms are growing in a circle in my yard. How do I get rid of them?
Questions to ask: Do the mushrooms grow in a complete circle or are they randomly appearing around the yard? Is grass in or around circle dead or alive?
Mushrooms that grow in a circular pattern in the yard are called fairy rings. They live on dead organic matter in soil or thatch. Usually there is dead brown grass just inside the mushroom ring. Fairy rings are difficult to control. Management strategies designed to mask symptoms include watering the soil, removing mushrooms, fertilizing properly and aerating soil. It is also possible to try and eradicate the problem by removing soil, fumigating and reseeding; however, this is usually not required. (Dial U Brief: Fairy Rings, 1990.)
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Stephen