Text adapted from Turfgrass Pest Management: A Guide to Major Turfgrass Pests in Oklahoma. Oklahoma Cooperative Extension Service.

CAUSE: A large number of thatch and soil-inhabiting fungi cause fairy rings in turfgrass. Most of these fungi produce mushrooms, toadstools, or puffballs of various sizes.

HOSTS: All turfgrasses.

SYMPTOMS: Fairy rings appear as small to large circular or arc-shaped rings of dark green, rapidly growing grass. A concentric ring of thin, dormant, or dead grass may develop inside the ring of lush grass ( Figure 1). The soil beneath the rings may become very dry and difficult to wet during summer and autumn. After rains or heavy irrigation during mild weather, especially in the autumn, the causal fungi may produce their fruiting bodies (mushrooms, toad stools, or puffballs) in the outer zone of lush grass (Figure 2).
Fairy rings may be very small when young. They may grow indefinitely, disappear unexpectedly, or be disrupted when intercepted by a street, a building foundation, or another ring. Fairy rings vary in size from 1 foot to 100 feet in diameter.

DISEASE CYCLE: How fairy rings begin is still unknown. The fungi begin active growth at a point in soil or thatch and grow radially outward. The initial appearance of fairy rings is usually a small ring or cluster of mushrooms, toadstools, or puffballs.

The fungi live by decomposing organic litter, which is abundant in the turfgrass thatch. The lushness of the stimulated zone occurs because the fungi release nitrogen as they decompose the organic matter in thatch and soil.

The zone of inhibition contains grass plants that may be dormant, mostly as a result of insufficient soil moisture. This ring of drought-stressed plants may occur because the extensive network of the fungi prevent water from infiltrating into the soil.

As the fairy ring fungi grow outward, they die out in the interior portion of the ring and release more nitrogen which can cause an inner ring of stimulated turfgrass.

FACTORS FAVORING DISEASE DEVELOPMENT: Fungi that cause fairy rings are common inhabitants of forest areas. These fungi begin growing on a source of organic matter, such as an old stump or large dead root in the soil, or excess thatch in turfgrass. Fairy rings usually cause more damage on light textured soils, drought stricken and nutrient deficient turfgrass.

CONTROL: Removal of large sources of organic matter, such as stumps and waste lumber, before planting will help prevent disease development. Power rake or vertical mow to remove thatch when it exceeds 1/3 inch. Golf course superintendents should regularly core cultivate and topdress golf greens to prevent thatch buildup. Fertilize to meet the nutritional needs of the turfgrass.

Coring, spiking, use of wetting agents and forcing water into the dry soil have provided some relief to this disease. Infected areas should be hand watered to prevent overwatering of the entire area which could prove harmful to the uninfected turfgrass.

More drastic methods of control involve soil fumigation, soil removal or turf renovation by rototilling and mixing the soil and replanting. Some fungicides drenched into the soil may control fairy rings; however, only limited success has been obtained because of difficulty in achieving uniform distribution of the fungicide through the entire soil profile colonized by fairy ring fungi.

Figure 2. -- "Fairy ring showing typical stimulated grass and fungal fruiting bodies", Oklahoma Cooperative Entension Service