It looks like lawn rust. This site says to fertilize and water longer and deeper

https://www.greenlawnfertilizing.com/library/lawn-diseases/lawn-rust/

 

Lots of information on rust on the internet. Read up. Most will say proper lawn maintenance is the answer, not chemicals.

Rust is common. Rust spores are floating around all the time. But normally, your grass grows, and then you cut the grass, faster than the rust can multiply. My guess is your grass is not growing right now due to it being so hot and dry.

 

Here's a list of options.
http://www.sunset.com/garden/garden-basics/lawn-problems

 

Thanks for replies. SPS-1, your info actually makes sense. I've seen much online about fertilizer, but couldn't figure out how that would help. If fungus eats grass, growing more grass faster seems to be just giving it more food. I'll give fertilizer a try, hopefully that will take care of it. I've read somewhere to minimize watering and let it dry out a bit, but it hasn't rained for a week, and that definitely isn't working.

 

I think the idea is to get it to grow and get cut before the fungus can take hold. So likely more controllable in spring and fall.

 

You need to get the grass growing, and then mow off the top, and bag it, don't mulch (while you see rust).

Read up on fertilizing a drought stressed lawn, if that is your situation. Not a good idea to fertilize a drought stressed lawn.

Water in the morning, so that it dries up during the day. If you water in the evening, it stays wet all night and that is good conditions to grow fungus. And water few times, but deeply. Not frequent light waterings.