Fairy rings are caused by at least 60 different fungi. These organisms create rings in turfgrass that range in size from a few inches to many feet.
There are three types of fairy rings. Type I causes rings or arcs of dead turf. Type II infestations result in rings or arcs of rapidly growing, dark green grass. Type III fairy rings appear as rings of puffballs or mushrooms with no accompanying change in turf coloration. They typically appear after a rain.
The dead rings found in Type I are believed to be the result of extensive fungal mycelium growth, creating hydrophobic soil conditions. Essentially, turf root systems don’t receive water, become drought stressed and die. Dark green rings associated with Type II are the result of nitrogen release as the fungal organisms decompose organic matter.
Generally, fairy rings occur when turf is under-watered or under-fertilized. Soils high in organic matter or turf areas with thick thatch are also prone to this problem. This is because fairy ring fungi feed on organic matter originating with buried debris, dead tree roots or thick thatch.
Fairy ring is difficult to manage. Individuals with Type II fairy rings can apply nitrogen fertilizer to “mask” dark green rings. Lawns affected with Type I require a bit more work. One should wet the ring area, including turf that extends 18 inches either side of the ring, with a soaker hose and keep it moist for 4-6 weeks. Alternatively, one can use a root feeder to inject water on both sides of the ring. A third technique involves core aerating the ring, including 18 inches of turf on either side of it. Follow with an application of a wetting agent.
Some have had good results removing the turf in the ring area and rototilling the soil in multiple directions to thoroughly mix it. Irrigate the treated area to stimulate microbial development and speed decay of fairy ring fungi. Leave the treated area bare for at least a month before over seeding. Seeding is the preferred method of turf reestablishment as fairy ring fungi can accidentally be introduced on sod.
To prevent fairy ring development, keep lawns adequately irrigated. Water affected lawn areas deeply at the first appearance of any ring type. Core–aerate lawns at least once per year.