Apples are firm, round fruits with smooth red, green or yellow skin and pale flesh that grow on perennial trees in the rose family. They contain small amounts of many vitamins and minerals and are high in fibre and antioxidants. The chemical compounds in apples cause the white or ivory flesh to turn brown when apples are cut open and exposed to air. Bacterial and fungal diseases, pests, injuries and improper storage can cause brown spots on apple skins.
Many apples develop brown spots from fungal infections. Fruit rot from microorganisms in irrigation water causes apples to develop tan to light brown rotten spots up to one inch in diameter on the skin. Russetting caused by cool, wet weather, frost, fungus and mildew occurs mostly in golden apples, which develop tan, teardrop-shaped flecks on the skin. White rot is a fungal disease that causes brown, circular spots surrounded by halos. Scab is a fungus caused by warm, wet weather that causes brown, corklike surface spots on apple skin. Black rot occurs during cool summers, causing alternating brown and black rings around dark spots. Sooty blotch is a fungal disease that causes brown smudges on apple skins.
Moths generally cause more damage to the flesh inside the apple, but brown spots can develop on apple skins around the area moth larvae enter the fruit. Injuries on fruit skin from the codling moth and the lesser apple worm might appear as dark spots on the blossom end of the apple. Tufted apple bud moths leave tiny holes, irregular scars, brown-coloured rot or corking on apple skin near the stem. When leaf rollers that usually feed on foliage eat fruit, brown scars can form on the apple skin. Feeding by brown or dusky stink bugs cause depressed, brown, corklike spots on apple skins.
According to a University of California Extension specialist and pomologist, apple skins can develop disorders while in storage. Storage scald is a light to dark brown discolouration of the apple skin caused by oxidative damage that kills surface cells during cold storage, especially in apples harvested when they are immature and those grown in hot, dry climates. Bitter pit caused by low calcium in apple tissue appears as dark brown, sunken surface spots. Sodium burn can occur on apple skins treated with a liquid chlorine solution before storage, causing the peel to turn brown after several weeks in cold storage.