Fairy rings are caused by a diverse family of soil-inhabiting fungi called basidiomycetes. They typically appear as dark green circles in the lawn ranging in size from a few inches to 200 feet or more in diameter. Lush rings of turf are formed by the release of nutrients, in particular nitrogen, from the activity of the fungus living on organic matter in the soil. A circle of mushrooms usually develops around the edge of the infected area.
Fairy rings grow outward at the rate of 6 to 24 inches annually depending on grass, soil and weather conditions. They are most common on sandy soils that are low in water and fertility. Thick thatch layers also contribute to this problem.
Besides being unsightly, fairy rings can cause serious damage to lawns. Often the disease will produce a thick fungal mat that prevents water from reaching the grass roots below. Once the soil becomes dry it is very difficult to wet and the grass roots eventually die. The fungus may also deplete soil nutrients and, in some cases, will release a toxic by-product that can directly kill the turf within a ring.
Note: The name “fairy ring” comes from an old folk-tale. People once believed that the mushrooms appeared where fairies had danced the night before.
Practices that promote healthy lawns help to reduce the occurrence of fairy rings. Remove excess thatch and aerate compacted soils. Keep lawns well watered. Encourage beneficial soil microbes by top dressing with a humus builder such as well aged manure or finished compost. Once the disease appears it is very difficult to eliminate. There is no natural control for fairy ring. Most homeowners prefer to mask the problem, using fertilizers rich in nitrogen, rather than eradicate it. Mushrooms can be temporarily removed by regular mowing or raking. Fungicides have not proven to be effective. Your mantra should be; Fertilize, water, aerate and mow.