Pic is hard to tell but it looks a reddish color which would prob be a fungus. I've seen 2-4D work well with fungus.

Would that work as a preventative method? Is it available as a granular product? I'm unfamiliar with fungicides aside what little I've read on here.

Here's a closeup shot as well.

2,4-d is not a fungicide, it is an herbicide. If it worked against fungus, that would be news to the lawn community. Not sayin' it won't work, just saying it would be news.

Is it an illusion or is this area slightly mounded up from the area to the left? If it is mounded up, then drainage would be better from that area allowing it to dry out more between watering/rainfall. That could easily cause a problem in one spot versus another.

The obvious issue would be not watering deeply, once per week, throughout the summer. But then why would you have any grass if drought was the problem???

If you have not used any organic fertilizer in the past 3 years, I would suggest you do that to improve the health of your soil. Healthier soil can fight off disease better than sterile or deficient soil. The one I like for this year (due to cost) is alfalfa pellets (rabbit chow). You can get a 50-pound bag at your local feed store for around $12-$14. If this is a first time in 3 years, your first app should be 10 pounds per 1,000 square feet. I would follow that up in a month with a second app at 20 pounds per 1,000. 20 pounds is the normal rate, but if your soil is not already sort of healthy, it will take longer for the alfalfa to decompose and it can smell a little sour for awhile. If you have doubts that alfalfa is a fertilizer, here is a picture posted here at GardenWeb by mrmumbles in 2011. He had fertilized the spot with alfalfa in mid May and took the picture in mid June.

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Note the improved color, density, and growth. Sometimes a lawn with poor soil will crash under repeated applications of chemical fertilizers. The occasional use of organics seems to keep the crashing from happening. Quite often homeowners will try the alfalfa and stop synthetics altogether. There does not seem to be any problem with a continual organic regimen. I've been doing it since 2002.

If you think the problem is a fungal disease, the other thing you can try, on top of the alfalfa, is to apply ordinary, ground corn meal at the same rate of 20 pounds per 1,000. Results on this seem to vary, but it has worked for me every year. If I did not use corn meal I would not have a lawn at all. In my yard it stops the progress of the disease immediately and allows the new grass to come up disease free. If you are reluctant to use fungicide, and I completely agree, then corn meal is the only alternative we think we know about. Like I said, results with corn meal are all over the map but it works for me. I would not try the corn meal until after the 10 pound app of alfalfa. Corn meal is also an organic fertilizer, so it will prolong the effect of the organics.

If you normally winterize with a chemical fertilizer after the grass stops growing, you can still do that whether you go with a full organic routine or not. Lots of people who are otherwise all organic will do the final app of the year with a chemical fert like urea.

The mounding is an illusion.

I wish to overseed the rest of the lawn and begin an organic program

All the posts I've seen re. organic overseeding require some kind of top dressing. However, my lawn is 1 acre and I can't afford to top dress the whole thing

I do have a tow behind Core aerator I can use. What are the steps for over seeding to achieve maximum germination? I've heard so many variations but I wanted to get your opinions.

Overseed and not renovation, okay.

Mow low, dethatch down to about 1/8-inch deep, remove all the fluff from dethatching, seed, roll, water 3x per day for 3 weeks or until you have 80% germination of your target lawn grass.

If you want to seed rye, fescue, and Kentucky bluegrass, understand that rye will germinate in 1 week, fescue will germinate in 2 weeks, and KBG will germinate in 3 weeks. You will likely need to mow the rye before the bluegrass is ready to mow. If you want a blend, avoid blends with both rye and KBG. You can do rye/fescue or fescue/KBG but the triple blend can be touchy to get good results.

I always advise against topdressing unless it is obvious that someone is going to do it anyway.