Answer: The most common causes for brown spots in lawns are 1) dog urine, or 2) cold season grasses going dormant during the hot months of the year. If the brown patches are the same grass types as the rest of the lawn, then I would suspect dog urine. If the brown patches are different grasses than the rest of your lawn, I would suspect the grass is simply dormant and will green up when the weather cools. Brown spots caused by dog urine will not green up again. The salts in the urine are concentrated enough to virtually kill the grass plus the roots. If this is what has happened to your lawn, you'll need to rake out the dead spots and reseed.
Best wishes with your lawn.
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