1. Water deeply and infrequently. Deep means at least one inch all at one time. Infrequently means no more often than once a week in the hottest heat of summer.

2. Mulch mow at your mower's highest setting. For St Aug you can go ahead and weld or Gorilla Glue your mower so that nobody can ever reset the height.

3. Fertilize with something at least 3x per year. Do it once in the late spring before the summer heat hits and twice in the fall after the summer heat breaks. If you use organic fertilizer, you can apply that stuff all summer long with absolutely no ill effects. In fact your lawn will get better and better. The longer I use organics and the more I read the experiences of others, the more convinced I become that everyone should be using it as often as they can and as much as their budget affords.

Specifically for you, yes the lack of fertilizer is your main problem. Watering is probably a problem but you didn't give enough info to tell. If you want to fast track your lawn back to health, get the water straightened out and star applying alfalfa pellets (rabbit chow) monthly at a rate of 20 pounds per 1,000 square feet. In 2 months you will be completely startled at how good it looks.

To prevent fungal diseases in St Aug, NEVER leave ANY yard clippings on the lawn overnight. I finally figured out that was the problem I was having every spring. My wife would clip garden stuff and let it sit in a pile for several days until the trash pickup. Sure enough the grass was brown underneath. Not only that but the brown spread and spread until I caught it.

If you are looking for an organic control for your lawn fungal disease, I have always had success using ordinary corn meal at a rate of 20 pounds per 1,000 square feet. Apply over the entire lawn for a couple reasons. Reapply in 3 weeks regardless of how much better things look.

As far as watering, I have an inground sprinkler. In the past I would water multiple times a week but I stopped that ages ago. Instead I water 3o minutes per zone once a week max.

So corn meal and rabbit chow? I am guessing a feed store is the best bet for this or do the box shops sell it under a different brand?

You should measure how much water your system puts out in 30 minutes. Use a cat food or tuna can to measure it. One inch is a starting point. Work up or down from there.