Fairy ring is caused by mushroom-forming fungi which form circular or semi-circular rings of highly variable size. Depending on the fungal species, different symptoms can appear, but typically are simply darker green rings that surround lighter turf. Rings of mushrooms can sometimes appear as well. Fairy ring is typically a less serious disease, and certain fairy ring mushrooms can actually benefit your lawn.
Susceptible species: All turfgrasses.
Favorable conditions: Fairy rings can occur year-round. Dense thatch layers or topdressing with compost or other organic material that is not fully decomposed increases susceptibility to fairy ring outbreaks. Stress from inadequate fertilization and drought can increase the severity of an outbreak.
Chemical control: Fungicide treatment for fairy rings is usually not recommended for residential lawns.
Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org
Lester E. Dickens, Bugwood.org