Where do you live?

What has your watering schedule been for the summer and since? How often and for how long?

Have you gotten much rain over the summer?

I live in Jacksonville, Florida.

weekly 2 times 50mins each zone. I am seeing this issue after Hurricane.

The lawn has a disease caused by too much water. There are two things you can try. If you try one then you can't try the other, but that's not a big deal. I have always (since 2002) had good luck killing disease in St Augustine using ordinary corn meal. Apply at 20 pounds per 1,000 square feet and give it three full weeks to recover. If you don't see the end of the disease in 3 weeks with new grass coming in, then you can either reapply more corn meal or you can try a chemical fungicide. Corn meal works by attracting a beneficial fungus which feeds on the fungus causing the disease.

The second thing you can try is a fungicide that is safe for St Augustine. The problem with fungicides in the south is the temperature is supposed to be under 85 degrees for several days. That usually limits the time you can apply to January. I don't have a recommendation for a fungicide, because corn meal has always worked so well for me.

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. You have two things working for and against you. You have humidity that slows evaporation and you have sandy soil which allows the water to sink through the soil faster. You'll have to tune up the watering to suit your situation, but watering twice a week for 50 minutes is probably not right.

Thanks for your time and suggestions.