Problem: Fairy Ring
This condition is caused by one of several fungi that grow on organic matter in the soil. These fungi do not harm the grass directly but may inhibit water flow into the soil. The ring of darker green grass is caused by nutrients released as the fungus breaks down organic matter. If the lawn is low in nutrients, the darker area will be more pronounced in contrast to the paler grass around it. The fungus begins growth at a central point and grows outward at a rate of 1 to 2 feet per year, forming the circle. Mushrooms, which are the fruiting bodies of the fungus, appear when weather conditions are right for them.
Fairy ring is not a turfgrass disease and does not harm the lawn. It is very difficult to control, but its effects can be masked by fertilizing the lawn outside the ring well and using less fertilizer within the fairy ring so that all the grass will be dark green. If there is a region of pale green or yellow grass within the ring, aerate the lawn and water thoroughly so that water penetrates the soil surface.