Answer: Pictures wouldn't be nearly as helpful as a sample of the damage. Failing that, I can offer some expert experience: If the patches appeared suddenly, and if your lawn is otherwise healthy, I'd venture that the damage is neither disease nor insect related, but caused by either dog urine or accidental application of a vegetation killer. Insect and disease problems generally are random in appearance not circular or spiral shaped. Carefully inspect the dead grass in the spots. Those killed by the salts in herbicides or urine will be dead clear down to the roots with no signs of life at all; grasses killed by disease will show white or brown matter clinging to the blades or the roots; grsses killed by insects will have insects or insect frass (poop) in the sample.
If you have access to a Master Gardener Clinic or Cooperative Extension office, helpful folks there can examine a turf sample under a microscope and positively diagnose the problem.
Best wishes with your lawn.
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