Fairy rings, caused by a number of fungi, produce peculiar patterns in home lawns. Fairy rings often consist of a dark green ring or arc with an inner ring of dead brown grass. The size of the ring can range from a few inches to more than 50 feet in diameter. Following irrigation or rain, mushrooms often suddenly appear within the rings.
Fairy ring fungi live on dead organic matter in the soil and thatch layer. Fairy rings expand radially outward until the food supply is used up, the soil becomes too wet, or better-adapted organisms surpass the ability of the fungi to survive. The ring and surrounding area can be broken into three zones, although all three zones may not be visible (see figure).
Cross section of a fairy ring
As the fairy ring fungus expands and enters new soil or thatch, an outer zone of stimulation forms. In this area, excessive nutrients are released as the fungus decomposes organic debris. Grass growth is stimulated, resulting in a ring of luxuriant green grass. Inside the zone of stimulation, a zone of dead or dying (brown) grass may be present. In this area, a high concentration of thread-like structures (mycelia) composing the body of the fungus can often be seen upon close examination of the thatch and soil. The mycelia prevent water movement into the area, weakening or killing the grass.
A third zone may be present in some rings. This is another zone of stimulation, inside the zone of dead, brown grass. As the fungus leaves this area or dies, additional nutrients are released allowing for luxuriant green grass.
To prevent fairy rings, properly water, fertilize, and dethatch lawns. Fairy rings are most common on sandy soils that are low in water and fertility. Thick thatch layers also contribute to this problem. Eradication of fairy ring fungi is difficult and often impractical. Most homeowners prefer to mask the problem, rather than eradicate it.
Soil fumigation. There are no chemical controls available to homeowners.
Replacement of infected soil. The sod and underlying soil, in a band about one foot beyond the ring, should be excavated to a depth of one foot, depending on the extent of the ring. Discard the infested soil, being careful not to spill any on the healthy lawn. Add clean, sterilized soil in the hole and reseed.
Masking the problem
Removal of mushrooms. If mushrooms are the only evidence of fairy rings, remove them by picking or raking, and destroy.
Fertilizer. The dark green rings will be more obvious in nutrient deficient soils. Conduct a soil test and apply recommended levels of fertilizer. Do not over fertilize, as this will cause additional problems.
Special Watering. Reduce the water repelling nature of the ring by watering the area with a root- watering wand. Punch holes at least every foot in the yellowing or dying area and pump large amounts of water into the ground to a depth of 10-24 inches. Repeat frequently. Application of a surfactant (wetting agent) to the area may also increase soil permeability and help to lessen symptoms.
Soil Aeration. Reduce symptom severity by increasing water penetration and accelerating thatch decomposition.
Chad Behrendt, Crystal Floyd