Sunburn is caused by excessive evaporation of moisture from the leaves and fruit. In hot weather, water evaporates rapidly from the fruit and foliage. If the roots can't absorb and convey water fast enough to replenish this loss, the fruit surfaces exposed to the sun overheat and burn; in severe cases, the leaves turn brown and wither. This usually occurs in dry soil, but fruit and leaves can also burn when the soil is moist and temperatures are around 100°F. Drying winds, severed roots, and limited soil area can also cause sunburn. For more information about leaf scorch, see Leaf Burn and Leaf Scorch.
There is nothing you can do once the fruit has been damaged, but it is still edible. To help prevent further sunburn, deep-water plants during periods of hot weather to wet down the entire root space. (For more information about watering, see How Much Water to Apply.) Newly transplanted trees should be watered whenever the rootball is dry 2 inches below the surface.
Plant Care for Persimmon