Hi Judi! There could be several things that cause what you have.  In trying to eliminate causes, a question: when you take hold of the "dead" grass, does it come up in a clump in your hand?

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Hi Judi! So the good news is you don't have grubs otherwise the grass lifts right up because the grubs eat the roots.

There are several patch diseases (summer patch, brown patch, etc) that are caused by a fungus. Once the night temps get above 68 degrees, the daytime temps are above 80, and the humidity is high, the diseases show up (sound familiar??). To keep the disease in check, if you have to water your lawn, then only in the morning; don't mow too low; either use a low amount of nitrogen to fertilize or don't fertilize. Any type of fungicide would only be used as a preventative (in mid-May is the best time). The good new is that once night and day drop as well as the lower humidity then the patch disease looks better.

I hope this helps!