Someone elses idea's if you want to use chemicals.
Provided a link below.
http://en.allexperts.com/q/Lawns-725/St-Augustine-lawn-2.htm

My Experience

I have not used any treatments (bug / 'cides) for my grass in probably 2 years now. This summer I watched ALL of my neighbors grass get brown patches and so forth. I did not have a problem for the last 2 years.

I water in the mornings twice a week and fertilize (less then stated amounts) 3 times a year. If I find an area that looks like it is getting in trouble I will throw compost on it (Example cow manure compost).

I have always noticed the ULTRA-GREEN lawns are the fist to die in the summers. My point is the more chemicals you add to the lawn, the more problems it seems people get. I would either dig it up and replace if it is small, or maybe try some compost and treat the soil. Healthy grass will creep in. This has been my experience. And though there is some weeds (the grass keeps it in check - most weeds in your lawn are from poor soil / severe dry conditions), it is one of the best looking lawns in the neighborhood. Adding fertlizer would be a bad choice at this time, could further the problem, go compost or replace.

Here is a link that might be useful: Could help

We also water twice a week in the morning as needed and we haven't fertilized in a while. The problem is that it's hot, humid, and rainy so we have the ideal fungal conditions with no help from us. It's all over rather than in just spots so any spot treatment/replacement won't do anything. We don't have too many weeds, it's just the gray leaf spot.

I am having the same problem. I had to replace a section of my grass about 3 months ago. It was killed by a combination of chinch bug and fungus... In my experience .. no way to keep this grass without chemicals here in Florida.

My latest approach is to take a complete soil test (in my case fungus was cause by lack of manganese and wrong PH) and treat for whatever is wrong (lack or excess)

Hope it helps