Any chance you put down a wetting agent without sufficiently watering it in? Last week I took the leftover slosh of water and residual Penterra left in my measuring cup and, not thinking, tossed it out in the yard. Didnt water in at all and a day or two later had spots like that where it had landed. Those spots resemble what you're dealing with. Apparently Penterra DOES have some phytotoxicity if not used properly haha.
*IF* it's fungus, looks like dollar spot to me. I had a slight, but easily manageable outbreak towards the end of last summer. A possible sign to see if it's fungus would be to get out there early in the morning to see if there's heavy mycelium presence in the suffering area. If so, there's a chance you may have a fungus issue.