Fairy ring is caused by a number of species of mushroom-forming fungi including many basidiomycetes in the order Agaricales. Many of these fungi are poisonous and must not be eaten.
Fairy rings are circular or semi-cirular and can range in size from a few centimeters up to many meters in diameter. Symptoms can be variable, depending on the species of fungi, and include: 1) dark green rings with no dead turf; 2) dark green rings with a thin ring of dying or dead turf inside or outside the green ring; 3) rings of dying and dead turf with a hardened hydrophobic layer of soil that is difficult to irrigate; and 4) rings of mushrooms without a visible effect on the turf. Weeds commonly invade infested areas.
There are two types of fairy rings, edaphic (non-superficial) or lectophilic (superficial). Edaphic rings are generally formed by fungi that inhabit the soil underneath the turf. These frequently cause a hardened hydrophobic layer of soil above the areas of their active growth that prevents water penetration into the soil and often causes plant death. Rings of dead turf are frequently associated with this type.
Lectophillic rings are caused by fungi that inhabit the thatch layer and decomposed plant debris, litter, and old thatch. These rings are characterized by a slight depression of the affected areas surrounded by dark green rings of turfgrass where the fungi are breaking down plant material and releasing nitrogen. A hard hydrophobic soil layer is not associated with this type of fairy ring.
The fungus survives as a white mass of mycelia in the soil or thatch layer, or can be spread by spores dispersed by the mushrooms produced by the rings. In some cases, rings or mushrooms can appear in the turfgrass and cause no apparent damage.