From what you say it is obvious you love your yard. It also looks well manicured in the pictures. As far as what disease you have it could be a few. Close-up pictures of the grass blades would be helpful. The experts on here can tell you far more about which disease than I ever could. There are organic and chemical cures that can help you out. If you're not an organic person, your local big box stores or local garden stores will sell chemical products that should knock the disease out. They also help prevent disease in the future

While on the subject of preventing disease, the first thing I would tell you is to adjust your watering habits. Your yard needs roughly 1" of water per week (including rain), preferably all at one time. Set some tuna cans around the yard and see how long it takes your irrigation system to put out 1" of water. The best time to water is early morning. Watering in the morning when it is cooler lets most of the water get into the grass rather trhan evaporating in the heat. Once you give it that 1", don't water again until you see signs of stress (curling grass blades, graying color, etc.) It will bounce right back with that water.

The second watering you do in the evenings are probably adding to your disease problem. The grass should have time to dry before the sun sets as diseases love moisture, especially when it is sitting on the grass overnight.

Looks a lot like summer or brown patch, but you would have to take it in for a positive ID. The conditions that favor the progression of the disease are fertilization during warm months and frequent moisture or high humidity.

At this point, you can try to treat the problem, but you are unlikely to find much success. Once it spreads all over, you are generally stuck with it until the heat breaks. Most of the fungicides are only marginally effective. Organic treatments like cornmeal, milk or baking soda might provide a little relief, but they are best used early in the disease cycle.

In the longrun, you should adjust your fertilizing and watering practices the reduce the chances of this coming again next year. There is nothing wrong with Scotts if that is the brand you want to use, just don't follow their instructions for timing. Just fertilize once in spring and then hold off on fertilizer until the summer heat breaks. For watering, you are keeping the grass moist too much. This is helping fungal problems to get a foothold and spread. You will be better off increasing the length of time you water as opposed to watering more frequently. If you do have to water a few hot spots due to shallow soil, try to do that in the morning so the grass isn't wet all night.

Also, next year, you should take measure to prevent the problem from reoccuring. They sell fungicides you can use before you see signs of the problem. If you would like an organic solution, spreading 10 lbs of cornmeal per 1000 sq ft seems to keep a variety of fungal problems at bay.

If you have enough damage that you will have to overseed this fall, spend a bit of time researching the grass you use. Some show resistance to certain fungal problems naturally which will make controlling damage much easier on you.

Yeah, leave the Scott's step program alone. As Bill said, it's okay to use their products, but they have you fertilizing at the wrong times. Check my response in this thread for a brief explanation. It's the 9th one from the top. Bill described fertilizing in spring. Just to clarify, make that late spring like early - mid May. Yep, that dismisses the Scott's early spring (March/April) application. Theirs normally has the Halts addition to help prevent crabgrass. If you still want crabgrass prevention, Scott's sells Halts alone w/o fertilizer. Use that instead.

Three elements of lawncare to help prevent disease
Proper irrigation (as described above)
Fertilizer timing
Resistant grass varieties

There in Illinois, I expect your lawn is Kentucky Bluegrass. Please correct me if I'm wrong, and I'll see if I can compose a blend that is bred for patch disease resistance if you would like.

Thanks to all of you and your responses. I realize I have taken my sweet time in replying but I have been attempting to analyze and observe my lawn in light of your joint comments. I have learned quite a bit.

1. I had just applied my second (!) warm weather feeding before reading your posts so that was no good.

2. I have finally collected enough tuna cans and performed several tests and discovered to my utter dismay that I basically give my south facing zones approximately one inch every time I water! This may be obvious to most posters here but for me, I have continued in recent years to mistake the brown areas for dryness and yet they are the most watered parts within each irrigation zone. I had ceased evening watering immediately after your replies and began using my bird bath as a rough indication of rainwater but per your instructions I should only be watering once per week (when there is not significant rain) which is simply astonishing to me, given my prior perception.

3. After your posts, it was moderate temperature here for a couple of weeks low 80s at most and pretty dry and my lawn began to look a little better (I concluded it was the second fertilizer application kicking in) but in the last 2+ weeks there has been huge rainfall, 90 degree days and high humidity. The brown patches have spread wildly and some areas simply look like they are completely dead.

4. In response to bestlawn's seed inquiry, while I am unsure what I started with, I have been re-seeding all the dead spots each year with Scott's Premium Kentucky Bluegrass mix for sun and light shade. Given that these are the first to die that is clearly the lawntype suffering the most.

5. What I struggle the most with is one neighbor who does nothing to his lawn to my observation and leaves his sprinkler on all the time even during heavy rain (he has no rain-gauge) and yet much of his lawn is green compared to mine - thus the implication is that this is a fertilizer issue primarily as we share weather and I assume, grass type?

The biggest frustration is to put all this time and effort in and be so misguided all along. This being said, may I kindly request some advice as follows:

Clearly I need to adjust my watering as recommended, fertilizer type and schedule and grass when I reseed this fall.

If I wish to go organic, can someone point me to a how to site or location that simply lays out the steps as opposed to my reading numerous posts and trying to divine a plan from all the back and forth? As I have a dog and am less than pleased with all the time and effort I have sunk into Scott's products and then had to take a couple of days after each application to keep the dog off the grass, it would be a pleasure to use products that are totally non-toxic, assuming this is true of organic products.

Can someone suggest what type of grass-seed I should put down this Fall? Ideally the recommended organic fertilizer treatment, how-to and schedule will suggest when to seed as well.

I am deeply indebted to all of you for your advice and assistance.

Sincerely,

Tom