Looks like a fungus to me as the slope of your lawn is running down the direction of the outbreak. Also the outbreak is in a fairly standard ring shaped form you can a small ring in the bottom right hand corner but two bigger ones in the left and middle.
Treat with this stuff - it comes in earlier liquid or granular form (i prefer the liquid spray, as i can control the application a bit easier. The active ingredient is 'propiconazole'.
edit: actually it looks like 'Leptosphaeria korrae' - necrotic ring spot - which there is no cure for, however it should work itself out. read more here. Ignore my advice on the lawn fungus treatment and save your money.