Mushrooms are related to smuts, rusts, and other pathogens, and as such, are members of a large group of organisms called fungi which are devoid of chlorophyll. These fungi are active in the decay of buried stumps and other bits of wood which can contribute to the formation of fairy rings. They appear in clumps or singly; but when they appear in a circular pattern, they are called fairy rings. Although mushrooms are one of nature’s ways of recycling nutrients, when they develop in a well-manicured turf they are considered a nuisance. No turfs are immune to fairy ring problems. Those with heavy thatch or under stress are more prone to fairy ring development and subsequent turf injury.
The life cycle of fairy ring-forming mushrooms is similar to that of other common mushrooms. Fairy ring fungi survive as dormant spores or mycelium in the thatch and soil. The mycelium becomes active during moderate, wet weather; and a ring continues to grow outward each year. Following rains, mushrooms appear within the dark green ring or at the edge of the dead area. Fairy rings usually are most severe in light-textured, low fertility soils low in moisture. The mushrooms grow on decaying organic matter and are most likely to form in areas where trees have been removed or in turfs with a thick thatch.
Fairy rings are found in three general patters: (1) mushrooms appear in circles and last only for a brief time without the presence of a dark green ring; (2) grass growth is stimulated and a dark green ring, along with the presence of mushrooms, is produced; (3) circular patterns of dead grass develop in the center of the dark green ring along with the presence of mushrooms.
The dark green rings of stimulated grass commonly vary from 1 to 10 feet in diameter and are particularly visible on turfs yellowing from iron chlorosis and in midsummer on turf that is deficient in nitrogen or under moisture stress. The dark green grass is caused by a rapid release of nitrogen in the soil. A concentric ring of thin, dormant, or dead grass may develop inside the circle of lush grass as a result of drought stress caused by the dense mat of fungal mycelium present just below the ring.
Tree stumps, large roots, and pieces of construction lumber should be removed before a new turf site is sodded or seeded. In established turfs, symptoms are “masked” by pumping large quantities of water 10 to 24 inches deep into the soil at 1-foot intervals within the ring and at up to 2-foot intervals on either side of it. The procedure should be repeated every two to three weeks during the growing season. A light fertilizer application during the growing season will reduce the contrast in green color between the fairy ring and the rest of the turf.
If desired, the fairy ring can be physically removed by carefully digging out the sod to a depth of 12 to 18 inches in a zone 2 feet on either side of the dark green ring of grass and replacing with fresh topsoil. Another approach is to kill the turf in the infested area with glyphosate and rototill the entire area in different directions to mix the mycelium from the different rings. The area can then be seeded or sodded.
The fungicide flutolanil is registered for the suppression of certain fairy ring fungi. It could be used to spot-treat problem fairy rings in certain areas such as golf greens; however, suppression may be only temporary.
J. E. Watkins. (1997). Diseases of Cool Season Turfgrass. In F. Baxendale, Ph.D., & R. Gaussoin, Ph.D., Integrated Turfgrass Management for the Northern Great Plains (pp. 137 – 138). Location: Nebraska