Answer: This might be a fungus. It is very difficult to diagnose this type of problem long distance. I would suggest you work with your local county extension to obtain a specific diagnosis and based on knowing that, determine how to proceed. In the meantime, here is a fairly comprehensive web site on lawn diseases from Cornell University Extension you may find interesting although it often takes an expert to make a diagnosis.
I'm sorry I can't be more specific for you. Good luck with your lawn!
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