Apple skin russet is light brown in color and rough to the touch. Russetting may be limited to small areas of the fruit or may cover nearly the entire fruit. During early-season, cells in the waxy layer, known as the cuticle, are not expand adequately as internal fruit tissues grow and very small cracks may develop in the fruit skin. Cells under the cracks die and cork cells form. Cultivars with thin cuticles are most likely to russet. Most cultivars have crystalline wax platelets on the fruit surface, but ‘Golden Delicious’ is very prone to russet, because it has an amorphous cuticle. Russetting is caused by cork cells developing in the skin tissue of the fruit and appears as a brown rough surface.  Skin russetting does not affect flavor or eating quality of the fruit, but russetted fruit may shrivel in storage. Often rusetting is most severe around the stem basin, but occasionally the entire fruit surface may be covered with russet.  Some apple cultivars are prone to skin russetting.  

Russetting is a characteristic of some cultivars grown under certain environmental conditions and sometimes the severity can be exacerbated by spraying certain pesticides and it is usually not associated with diseases or insects.  ‘Golden Delicious’ russetting has been associated with wet conditions during the first 60 days after bloom. Skin russet is characteristic of ‘Golden Delicious’ grown in the humid east, but not in the arid west. The cultivar ‘Roxbury Russet’ is usually totally covered with russet.  Spraying apple trees with copper as a fungicide during the early season will cause russetting. Sometimes frost during bloom may not kill a flower or small fruit, but may injure the skin tissues and cause a ring of russet around the fruit and these are referred to as frost rings.

Some apple growers are noticing that some of their apples are looking a little strange, with odd colored scabby rings around the fruit. This is called frost ring and is a result of the freeze when apples are in bloom and the young fruits are just starting to develop. The freezing temperatures damage the surface tissue, resulting in a ring pattern as the fruit grew in size. Frost ring does not affect the taste of the fruit so the apples are perfectly edible.

Frost ring on apple

Skin russetting of 'Golden Delicious'

Refernces:

https://pnwhandbooks.org/plantdisease/host-disease/apple-malus-spp-fruit-russeting；