St. Augustine is a grass type that is well known for its susceptibility to lawn disease. In fact, the problem is so bad that their are lawn diseases that are unique to this type of grass only! That should say a lot. Take all root rot St Augustine grass disease is one of the worst diseases your lawn can contract. It literally spreads and destroys your whole lawn slowly but surely. It works its way subsurface in the root system. All of the roots begin to rot and slowly wither away. You can tell if your lawn has this disease by going to some of the dead spots and pulling up on the grass. If the dead grass plant pulls away as if there was no resistance at all, then you have got the deadly TARR disease. There is only one control for this disease and it isn’t a panacea either. You can apply peatmoss topdressing as a control, but it won’t cure the lawn unfortunately. You might have to remove all the dead areas and start over with a new lawn establishment.