These are pictures of Summer Patch on Kentucky bluegrass at the Horticulture Research Station. It is caused by the fungi Magnaporthe poae. It typically shows up in early summer, particularly in years like this that are very wet early followed by a quick drying period and hot temperatures. This showed up over the 4th of July (right on time).
The blighted areas with a green center that are surrounded by a circle of dead grass are known as “frog-eyes” and are typical of a number of patch diseases. It is believed that the organism begins as a saprophyte (organism that feeds on dead plant material) in the middle and moves outward in a circle without damaging living grass. It only attacks living grass if conditions are right and it reaches a certain level of virulence. The patches here are 10 to 12 inches in diameter, which is common for this disease.
There are several systemic fungicides labeled for this disease, but the trick is to get them down before the symptoms develop. Contact fungicides will not work. To treat now would do no good and the symptoms will like last through the summer. This is a disease for which good records are a must. On this area, I would need to apply a systemic fungicide in late June next year before symptoms develop. Core aeration in the fall and irrigation during the stress period of early summer can also help prevent its development.