grub worm, heat, or disease damage

My grass is starting to develop patches throughout the lawn and not sure what it is or how to treat it? Initially, I had one patch in the backyard which has grown and new patches are starting to scatter throughout the entire lawn. I have only laid milorganite (about 3 weeks ago) and scott's weed and feed (quite a few months ago). I haven't had time to actually dig up to check for grub worms but if I don't find any, could this be brown spot?
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grub worm, heat, or disease damage