Sun scald is not a disease, but damage caused to apples and many other fruit, caused by high temperatures and strong sunlight in summer.
Typically, sun scald damage appear on sun-exposed side, which is usually towards the direction of the midday to afternoon sun. The cells die in the sunburnt area, forming an irregularly shaped reddish-brown patch which is sunken below the surface of the fruit. The flesh underneath the damaged area also turns a brown colour. Fruit with sun scalded damage tend to drop prematurely.
Fruit are more likely to be damaged by sun-scald in hot, dry weather with continuously strong sunlight, especially in young trees where there is not much foliage covering the fruit. Heavy pruning which reduces leaf cover over fruit will cause sun scald, as will poor quality soils which don’t retain moisture, causing the tree to dry out and burn more easily.
The best way to prevent sun scald is by shading, having some form of protection from the hot west afternoon sun. This can be in the form of dappled shade provided by other trees, or by frames and structures supporting shadecloth (use 50% shading rated shadecloth for plants and trees, higher ratings are for humans only, and don’t allow sufficient light through for plants).
Ensure that trees receive adequate nutrition so that they can put out adequate new leafy growth to protect the fruit, feed with a balanced fertilizer in at the start of spring and autumn.
Mulching the soil in late spring to prevent water evaporation from the soil is very helpful in maintaining fruit trees in general, and can reduce damage to fruit due to moisture shortage.
Another method for preventing sun scald in extremely hot dry weather is increasing the humidity around trees. This is achieved by watering the soil surface (don’t water the leaves as this may promote fungal diseases!) with a hose, using a gentle spray. The cooling effect reduces the effects of the heat and the risk of fruit damage.