What are those dark circles on the Greens?

These circles are created by a disease call Fairy Ring. Fairy rings are usually first noticed as irregular rings or arcs of darker green grass on the turf surface (Figure 1).

Figure 1

Figure2

The darker color is caused by the release of nitrogen as the fairy ring fungus breaks down organic matter. The margins of the rings are often irregular or wavy. On putting surfaces especially, the entire area defined by the ring may be darker green than the surrounding uncolonized area (Figure 2).The rings can remain green or progress to the point where dead grass develops at the margin (Figure 3).

Figure 3

Death of grass at the margin has been attributed to moisture stress due to hydrophobic soil conditions that develop as the fairy ring fungus grows. The death has also been linked to excess nitrogen release or the formation of hydrogen cyanide or other toxic metabolites. With some fairy rings, death of the grass also occurs in spots where the fruiting bodies have emerged and then collapsed (Figure 4 top). Fairy ring with resulting loss of turf is one of the most destructive diseases occurring on putting surfaces.
During these hot and dry periods, we have noticed more and more dark circle's showing up on our greens surface.  We are currently in the process of trying several procedures to eradicate the dark rings and to stop any more turf loss due to this disease.  We have punched small holes into the greens surface to not only help with oxygen exchange but to also help penetrate into the ring itself.  After punching the small holes we have then applied an application of ProStar which is a fungicide that is labeled to help control the symptoms of Fairy Ring.  We will monitor the progress of these rings and follow up in two weeks with another application of fungicide to help alleviate the problem.