Russeting of apples occurs when a corky substance forms on the skin of the fruit. Its pattern will depend on its causes which are many. Certain fungi and bacteria, wet weather conditions, copper fungicide sprays, frost and nutritional problems can all cause russeting. Two fungi which cause severe russeting are Aureobasidium pullulans and Rhodotorula glutinis, both of which are commonly found on apples. Infection usually occurs beginning at bloom and lasting for about a month after petal fall, especially if there is cool, wet weather.
Some apple varieties seldom have russet, others always have a certain amount of russeting around the stem, while there are others like Golden Russet which may be completely covered with russet.