Have you ever seen a large ring of darker green grass in your yard? This ring can be anywhere from a handful of inches across to many, many feet across. However, there is no dead spot in the middle, so it most likely is not dog patch, but instead it is fairy ring. What causes fairy ring, and what can be done about it?
Fairy ring is caused by fungus decomposing organic matter in the soil. Things like dead tree roots, or old buried stumps will become a feeding ground for fungus. Now the fungus is not harming the grass to make it so green. What the fungus is doing is breaking down organic matter and nitrogen is a by product, and nitrogen causes grass to green up. The fungus is not even interested in the grass, the grass just receives the by products.
Now what can be done with fairy ring? Since it does not harm the grass, there is no reason to treat the lawn with fungicides to get rid of the fungus. This fungus is a beneficial part of nature by breaking down dead organic matter. Fairy ring is just an aesthetic problem. The best way to handle the darker ring is to fertilize your lawn to green it up to try and mask the darker ring in your lawn. Just remember to be careful not to over do it and poison your lawn either by adding too much nitrogen. If the fairy ring is not in a highly noticeable area, it may be best to just let mother nature run her course.
Since fairy ring does not harm the grass, the concern level should be lower as it is just an aesthetic problem. Your lawn will survive fairy ring. And over time, whatever is there will completely decompose, and the problem will go away.