Answer: If you've fertilized regularly, my only suggestion is there may be a fungal disease at work. I can't diagnose it without a hands on inspection so your best bet would be to dig a sample (6" x 6" with roots) and take it to your local cooperative extension office for diagnosis. If they can't tell through microscopic examination they can send it to the plant pathology lab. I'm not sure which county you're living in so here's a list of cooperative extension offices listed by county:
Best wishes with your lawn.
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