Answer: If your soil doesn't drain well or if you keep the grass overly wet, it can develop a fungal disease. More likely you're seeing damage from dog urine. In either event, I'd recommend raking out the dead grass plus the roots and then reseeding. Grass seed germinates in 7-10 days and within a month should green up and fill in those bare spots. If your soil remains wet for several hours after watering, you're either leaving the sprinklers on too long, or the soil is compacted and won't drain properly. You can avoid over-saturation by running the sprinklers for shorter periods of time.
Best wishes with your lawn!
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