Where do you live?

What is your watering schedule? How often and for how long do you water?

Are the problem spots in the shade?

Are the problem spots in low spots?

Does anyone, or has anyone, leave anything on the grass for longer than a day? For example other yard clippings tossed on the grass and left overnight or longer until they can be disposed of?

I live in Jacksonville, fl. I water twice a week about 1/2" each time. I increase during the hot and dry times. There is a little shade from one tree in the AM only. Lawn is level and there is nothing left on the grass. I've even monitored for dogs urinating in those areas but haven't seen any.

I'm going to assume it is a fungal disease. Your watering is wrong enough to possibly get a root rot going. Would you mind posting a picture? The best garden pictures are taken on an overcast day to reduce the contrast between brights and shadows. For this the distance should be about 6 inches away.

I have had a similar disease issue in my St Aug where it returned time after time, but mine was a surface disease and not affecting the roots. Mine did not go away until I treated it with corn meal - yes, the same stuff you eat. Apply at 20 pounds per 1,000 square feet over the entire yard. You can buy it in 50-pound bags at a feed store. Call first to see if they have it. Ask for the price. It should be well under $15 and more like $5. If the price is above $20 then it's not corn meal but corn gluten meal. That won't work. You want ordinary corn meal. Cracked corn will work, but don't use whole corn. Whole corn will turn your yard into a corn field (easily mowed down).

Corn meal works biologically and not chemically. The corn decomposes with bacteria and fungi already living in the soil. One of the fungi that seems to accumulate especially well on corn is Trichoderma (try ko DER mah or trick oh DER ma). This is a predatory fungus that feeds on the disease causing fungus. Give it three full weeks to show improvement. If it seems like it is working, but you want to really hammer it home, reapply.

If you have already used a chemical fungicide in the past few weeks, then corn meal won't work. Why? Because the Trichoderma has been killed off by the fungicide.

In the future you should change your watering. I realize 99% of all Floridians water daily, but they are all wrong. All of them including the sprinkler guys who set everyone up. Here's more about watering.

Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the daytime air temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall.