I have 3 patches of gray, ash-like substance in my St. Augustine grass. It looks like someone poured the contents of their fireplace ashes on it. I also have some tunnels that may be moles or voles. The patches are very hard and powdery at the same time. I can scrape down to the grass which looks yellow and sparse. Haven’t a clue – do you?
I’m not familiar with diseases on St. Augustine grass since we don’t really grow it here in Upstate NY. A google search turned up this. Hopefully this will help. Here, we have Mole-Max which will drive soil dwelling rodents away.