In addition to the green ring, which may make the lawn look uneven, there is sometimes a bare area either on the outside or inside of the green ring itself. This is caused by either the fungi releasing a temporary toxin, or the mycelial mat itself repelling water. If you take a pick to the edge of the ring, look at the soil clods that come up. This is the underground body of the fungus. Sometimes, but not always, you may see toadstools associated with the ring along the edges.
Toadstools occur under ideal temperature and moisture conditions.
Sometimes you may see a ring of toadstools, but NO green ring. Unsuccessful remedies include throwing boiling water on the ring, adding motor oil with water, or pouring vinegar on the ring. Some people will go to the task of digging out the ring and replanting. Don't try these. The added nitrogen from the fertilizer makes the surrounding grass green.