Something that comes up from time to time, especially in the Uk is Red Thread Disease.
Unfortunately the pathogen which causes Red Thread is within the species of certain grasses, so first, ensure that you purchase your seed from stockists who have a high rating free from Red thread certification.
Secondly never cut the grass too short, it is in this condition, when it is trying to regrow from this weak position, that the disease will spread most rapidly. It rarely kills the grass plant, rather just exhibits its red brown withered leaves. Usually around 10cm clumps are found, but do spread and can join up with others , where it looks really unsightly. The best advice is to set the cutting height to no less than 1.5 inches in mid summer and never cut an overgrown lawn more than 1/3 of the total length , in one go. It just gives the grass time to recover.
Lastly, in the UK we tend to be advised to use fertilizer in the spring. So by mid summer, when we are mowing and have the full heat of the sun, perhaps not watering as often as necessary, the benefit of the fertiliser is running out, this too will promote Red Thread, so apply your fertiliser over a longer period, to avoid this.
Make certain that you are using good fertilisers and not some garden growing chemicals with pesticides, I was alarmed to see that some of the worst, were still being sold despite all the studies about these known carinogens getting into our water tables