Fairy Rings (fungi - Agaricus spp., Marasmius oreades): Fairy rings grow in circles or crescent-shaped areas, ranging from a few inches to 50 feet in diameter, feed on organic matter in the soil and thatch layer. Areas previously covered with trees or fill which contained stumps or logs are prime candidates for development of fairy rings. Fairy rings are a common problem on newly established golf greens that contain a high level of organic matter. Fairy rings are either dark green or brown. Brown rings develop when fungal mycelium forms a hydrophobic layer. This layer prevents water from reaching turfgrass roots, resulting in drought stress. Turfgrass next to the ring may be dark green because of nitrogen released from organic matter on which the fungus is feeding. Mushrooms may or may not develop after a period of heavy rainfall or irrigation.
Management: Vertical mowing and topdressing to reduce thatch and removal of tree stumps and roots reduce the organic matter on which the fungus feeds. Fertilization may mask dark green fairy rings by stimulating growth in the rest of the turf. Aeration and drenching the soil with a wetting agent will minimize development of the zone of brown or dead grass in the area of dense mycelial growth. Fairy rings are difficult to control with fungicides since soil in the infected area is almost impervious to water. Sporadic success has achieved by aerating and drenching with fungicide.