Scald occurs when the leaf temperature reach the max temperature the plant can withstand - then cell walls break down, organelles stop functioning and the leaf blade dies. The most common occurrence of "sun scald" on St Augustine turf is during hot dry weather when mowers with hot tires, travel across a hot lawn on a hot day and cause scald. The grass will recover but it takes time.
Sun scald can appear as a "pock marked lawn" with small 6" circular areas of live grass in a larger area of tan/dead turf in sunny spots in the lawn. These areas will recover when there is adequate moisture and cooler temperatures.
The solution is to keep the turf cool by keeping the soil moisture at medium levels (field capacity). The evaporation of water from surfaces and evapo-transpiration processes of plants cool the plant down. No water – the grass overheats and dies.
Tip: A cooling midday watering can help – it is a myth that watering during the middle of the day damages grass or is a waste of water. A light sprinkle of water on the lawn is like jumping into a pool for the lawn – it cools and refreshes. Also, if possible mow during the cooler times of the day and mow at the highest setting on your mower.