Fairy Ring
A Problem of Turfgrass

Curtis Swift, Ph.D.
Colorado State University Extension
Tri River Area (Horticulture)

A Fairy Ring Infected lawn - early Spring

Class: Basidiomycetes
Genus and species: many

Fairy rings are reported to be caused by many (60) different soil-inhabiting fungi of the class Basidiomycetes. These fungi can cause the development of rings or arcs of deep green grass as well as unthrifty or dead grass. Rings may vary in size from a few inches to 200 feet (60 meters) or more in diameter with an annual radial growth of 3 inches (7.6 cm) to 19 inches (48 cm) depending on grass, soil and weather conditions.

The term `fairy ring' has its origin in myth and superstition as they were believed to be the result of a circle of dancing pixies (`fairies'). These circular rings were also thought to be the result of lighting strikes and where the devil churned his butter. Today's research community has shown the dark green circles are the result of fungi colonizing the soil, leaf litter or thatch. The break down of organic matter by fungal activity releases nitrogen stimulating grass on the outside of the ring causing it to grow taller and darker than surrounding grass.

The band of stimulated grass is often associated with the fruiting bodies of the fungus. The fruiting bodies range in size from 3/8 inch (1 cm) to 12 inches (30 cm) in diameter. Some of these so-called `mushrooms', `toadstools' and `puffballs' are poisonous and are best picked and disposed of if young children frequent the area. Removing these fruiting bodies does not weaken the fungus but does help improve the aesthetics of the area.

Fairy ring starts from a piece of mycelium or spore at a single point feeding as a saprophyte in the thatch layer or on soil organic matter. The uniform outward growth of the fungus results in the development of rings. Changing soil types, the fungus involved, condition of the turf, abundance and type of organic matter and obstructions all affect this radial growth. Fairy rings encountering each other in their development will typically produce a scalloped effect of stimulated or dead grass. This condition is thought to be due to chemicals (metabolites) produced by the fungus inhibiting the growth of other fungi. This inhibition is called `fungistatis'.

A similar condition is thought to occur on slopes, where the lower part of the ring breaks possibly due to the downward movement of these self-inhibiting metabolites. These substances are thought to prevent the growth of the fairy ring fungus in the turf at the lower part of the ring.

Under certain conditions, and with certain fairy ring fungi, a ring of dead grass develops. Some of the responsible fungi have been shown to penetrate and kill root cells resulting in dead rings of grass. In addition, the mycelium of some fairy ring fungi are reported to be hydrophobic, creating a water-impervious layer resulting in drought-stress problems for the grass. Once the soil under this mycelial layer becomes dry it is very difficult to wet and the roots of the grass plant die.

Some fairy ring fungi stimulate the grass but do not cause its death. The development of other fungi result in no stimulation or damage of any sort. Instead, a ring of mushrooms is the only indication of the presence of the fairy ring fungus.

Depending on environmental conditions, several years may pass without the production of mushrooms. The presence of dark green or dead rings may themselves be lacking. Turf subjected to extreme drought stress is more susceptible to problems from fairy ring.

Attempting to change soil pH does not appear to have a controlling effect as fairy rings have been observed on soils with a pH ranging from 5.1 to 7.9. Fairy ring develops most frequently in soil high in organic matter, or in lawns with thick layers of undecomposed thatch. Consequently core aerating to remove thatch and enhance its breakdown as well as other thatch management efforts are recommended. When preparing soil for seeding or sodding, remove any large sources of non-composted organic matter such as tree stumps, wood building materials, etc. These provide a food base from which fairy ring fungi can spread.

The application of fungicides is not a recommended control option provided by turf pathologists The use of pesticides may increase the incidence of fairy rings by destroying saprophytic bacteria and fungi which compete with the fairy ring fungi.

Ineffective control of fairy ring by fungicides may be due, in part, from the hydrophobic nature of the mycelium preventing chemicals from reaching those portions of the fungus 2 to 3 feet (0.6 to 1 meter) below. Drilling holes prior to applying fungicides in hopes of counteracting this problem is also reported as ineffective.

Several researchers recommend fairy ring infected turf areas be fumigated. Watschke, et. al. recommend sod be removed two feet on the inside and outside of the ring, the soil loosened to a depth of 6 to 9 inches, a soil fumigant applied and the treated area covered with a plastic tarp. Depending on temperature, fumigation may be complete within two weeks. Fumigants can be caustic, burning through clothing, skin and lungs. Use with extreme care or hire a professional applicator.

A second option involves removing the soil to a depth of one foot and wide enough to extend at least two feet on either side of the fairy ring. Replace this soil with non-infected soil and reseed or lay sod.

Note: If any of fairy ring fungus contaminated soil or sod is spilled on healthy grass during either of these processes, the fungus will start over in that location.

A third option involves killing the sod and then rototilling the fairy ring infected area in several directions until the soil is thoroughly mixed. This method of control takes advantage of the self-inhibiting metabolites produced by the fungi. It is thought that mixing these materials together will prevent the regrowth of the fairy ring. Success has been reported with this method.

Continual soaking of the soil for four to six weeks has been shown effective in eliminating fairy ring. This involves keeping the soil saturated in a band 18 inches (45 cm) on either side of the fairy ring. A soaker hose or soil needle can be used to accomplish this task. Perforating the watered area with a pitchfork or garden fork to a depth of about 6 inches (15 cm) helps water penetration. This technique should be avoided when fairy rings are numerous and tree and shrub roots exist within the proposed saturated soil area. Root death of the woody plants may occur creating problems for trees and shrubs.

In Colorado, applying nitrogen masks the symptoms of fairy ring by causing the rest of the lawn to green up. This effectively hides the deep green ring caused by the saprophytic fungus. Aerating the fairy ring with a core cultivator (core aerator) and utilizing a wetting agent to help penetrate the hydrophobic mycelium improves water penetration. This has been shown effective in preventing the development of dead rings. Using a soil needle (deep root feeder) to aerate and irrigate dead and dying rings is also recommended.

Beard, et. al. report with some fairy ring mushrooms, fertilizing results in an increase in fairy rings.

References used:
Ali, A.D. & C.L. Elmore. 1989. Turfgrass Pests. Extension University of California. Publication 4053.
Beard, J.B., J.M. Vargas, Jr., and P.E. Rieke. 1973. Influence of the nitrogen fertility level on Tricholoma fairy ring development in Merion Kentucky bluegrass (Poa pratensis L.). Agron. 65:994-995.
Couch, H.B. 1995. Diseases of Turfgrasses - Third Edition. Krieger Publishing Company.
Leslie, A.R. 1994. Handbook of Integrated Pest Management for Turf and Ornamentals. Lewis Publishers.
Smiley, R.W., P.H. Dernoeden & B.B. Clarke. 1994. Compendium of Turfgrass Diseases - Second Edition. APS Press.
Vargas, J.M. Jr. 1994. Advances in Turfgrass Science: Management of Turfgrass Diseases - Second Edition. Lewis Publishers.
Watschke, T.L. P.H. Dernoeden, & D.J. Shetlar. 1994. Advances in Turfgrass Science: Managing Turfgrass Pests. Lewis Publishers.