What type of organism causes fairy rings in lawns?

Fairy rings in lawns are primarily caused by mushroom-forming fungi. These fungi grow in circular patterns, creating visible rings of lush grass or mushrooms, depending on the stage of growth. The mycelium of the fungi spreads outward from a central point, depleting nutrients in the soil as it grows, which leads to the characteristic ring of vigorous grass growth or a ring devoid of grass where the fungi have consumed the organic materials.

The other types of organisms listed do not cause fairy rings. Bacteria are involved in various soil processes but do not form the visible, complex structures associated with fairy rings. Viruses primarily affect plants at the cellular level but do not create the growth patterns seen in fairy rings. Nematodes are microscopic roundworms that can impact plant health but do not form the distinctive circular patterns associated with this phenomenon. Therefore, mushroom-forming fungi are the key players in the formation of fairy rings in lawns.