What Makes an Orange or an Apple Rot?

Apples and oranges are among the most favored fruits for nutrition, flavor and convenience. Each type of fruit regardless of the individual variety shares a reasonably long shelf life and is considered sturdy under normal conditions. However, as organic matter, both the apple and orange are subject to rot under a number of conditions.

The cellular structure of an apple ruptures when pressure is applied, which results in a bruise. Bruising begins a rotting process. Oranges are also subject to injury, even with their notably thicker skins. Injury can occur while on the tree by either insects or extreme conditions. Too much water absorption causes the skin on a fruit that is no longer growing to split. Injury after harvesting is usually due to mishandling, improper storage or insects.

Texas A&M University notes different varieties of apples are subject to different fungi or diseases, such as apple scab, blister spot and bitter rot. Brown rot and black rot are a fungus, usually the result of injury while the apple is still on the tree. Washington State University warns that infected fruits, though properly refrigerated, may still develop active decay at a later date when the apples are relocated into warmer temperatures. Oranges develop blue mold, also known as storage rot, “the most feared storage disease of citrus fruits,” according to Transportation Information Service (TIS). “The fungal spores penetrate through small injuries” and are transferred by contact from one fruit to another.

Once insects pierce the flesh of the fruit, the rotting process begins. Insects affecting apples during growth include red mites, coddling moth and apple maggots, while the problem insects for oranges include aphids and mealy bugs. Mediterranean fruit flies bore into oranges, which causes rotting and gives the fly a location to lay eggs. TIS notes that International quarantines are in effect against this insect, requiring all affected fruit destroyed.

Dry storage is necessary for both the apple and orange to prevent rot. Excessive humidity encourages bacterial and mold growth, which penetrates the skins. During transport, TIS cites that the fruit must be protected from all forms of moisture, including condensation as all moisture “promotes green and blue mold and black rot.”

Apples and oranges emit gases that aid in the ripening process of the original fruit and other nearby fruits. Even after fruit has been harvested, it continues to "breathe" in oxygen and emit carbon dioxide and/or ethylene gas. Apples produce ethylene gas, according to the Virginia State Apple Board, which can be used to help other fruits ripen more quickly. TIS recommends maintaining a “continuous supply of fresh air” otherwise “citrus fruit can start to ferment within a few hours due to anaerobic respiration,” resulting in rotting fruit.

Although rare, rapid temperature swings damage apples and oranges while on the tree, but the greatest threat occurs during transportation and storage. Apples require a cool temperature for storage and do well under refrigeration. Oranges are cold-sensitive, but can withstand refrigeration so long as the pulp is not less than 36 degrees Fahrenheit, according to the Ohio State University Extension. Both fruits sustain cellular damage and begin to rot after introduction to freezing temperatures or excessive heat.