Looks like brown patch. The small brown lesions that are starting throughout the blade is a good indicator.

Has the grass been really damp/wet for long periods of time?

 

This past week in st louis has been warm and humid. Got some over night rain either weds or thurs. I mowed fri afternoon and areas of the lawn where still wet. Got a lot of rain yesterday and was pretty much cloudy all day. Lawn is pretty wet today.

 

If I were to guess, I'd put my money on brown patch, especially with the late night rain, humidity,etc. perfect breeding ground for fungus. Scott's diseasex has the active ingredient azoxystrobin that should combat that with a curative rate.

 

Thanks @Scagfreedom48z+. How you like mowing with that scag? I have a 36" ferris walk behind, but I'd like to try scag at some point. Most lawn companies around here seem to run ferris and exmark.

 

Those are all great brands. I love my Scag. Cuts wonderfully and it's a treat to mow with. Very low center of gravity, powerful and American made. Built like a tank. I have a sloped hill in my front yard and it climbs like a goat. That being said, I always mow when the grass is dry and I know my limitations. I've had a few instances where the grass was a little damp and lost slight control on the way down the slope. Nothing major but lesson learned