Most fairy rings are caused by fleshy fungi such as mushrooms, toadstools, or puffballs. They occur wherever grass grows and in soil that is high in organic matter. The rings are usually marked by mushrooms or by a stimulated or depressed growth of vegetation.
There are three types of fairy rings. One produces a ring of dead grass each year and is visible for long periods of time. Another produces a dark-green ring. The third type is visible only when the fruiting bodies are produced and does not affect the grass.
The killing or dark-green reaction sometimes results from hydrogen cyanide (HCN) production, which in high concentrations is lethal to grass. Mushrooms may appear periodically. The soil is permeated by a white fungus growth (mycelium) and is usually very dry.
Fairy rings spread outward a few inches to a few feet per year. Where the grass cover is not uniform, the rings may not be completely circular. As the fungus spreads, it consumes a portion of the organic matter in the soil. The fungus prevents the penetration of water into the soil, and the grass is damaged or killed by drought.
In developing your lawn, do not bury roots, stumps, branches, and other large pieces of organic debris. These provide a food source for these fungi. If the only effect of the fairy ring is a stimulation of growth in rings, fertilization usually takes care of the problem. Removing cores of soil (aerif1cation) aids in water penetration.
Topdress holes with fresh soil free of organic matter. Some experts recommend removing the soil 18 inches on each side of the outer stimulated area 1 foot deep. In removing infested soil, do not spill it on adjacent healthy grass. Fill the trench with fresh soil and reseed or resod. Chemical control is generally not very effective because the fungus grows so deeply into the soil.