Mold on oranges seldom springs from the kitchen where it shows up. Whether stored in the open at room temperature or in a refrigerator, oranges carry mold spores from the orchards and fields where they grew. The most common molds are green and blue molds -- Penicillium digitatum and Penicillium italicum.
Fungus infections spring from the ground in mold spores that thrive in the climates where oranges and other citrus grow. The molds grow in the dark of shipping boxes as they travel through the warm, humid countryside on their way to market. Once in the produce bin in the market and then in the kitchen, mold spores travel to other fruit and vegetables. Molds gain a foothold in oranges through wounds or bruising at harvest, through transit or in the market.
The most common molds are penicillin species that begin as fuzzy white spots on the orange. Once established, the molds propagate aggressively. Green mold grows olive green from the center out; blue mold, like the green, grows inside a ring of white mold. Green mold can envelope an entire orange in two to four days. Blue mold reaches its peak at the end of the growing seasons when temperatures decline; it also survives easily in refrigerator fruit bins where temperatures average 40 degrees Fahrenheit.
Several other molds less frequently affect citrus fruit. Two of these, sour rot and black spot, also inhabit soil in orange groves and travel to oranges on splashing water or at harvest time. Like green and blue mold, they afflict orange rinds when the skin is damaged at harvest or by as minor an injury as that caused by a hungry insect.
Sour rot, caused by the fungus Geotrichum candidum, often coexists with green mold. It thrives in warm fall days. A light to dark yellow spot of mold turns slowly into a yeasty or wrinkled white mold that becomes slimy and watery. Citrus black spot, a compound of Guignardia citricarpa and Phyllosticta citricarpa, causes small black or reddish-brown blotches scattered irregularly around the orange.
Many mold spores survive easily at refrigerator temperatures and lodge in shelf liners and on surfaces, spreading to other fruits and vegetables. Only when they bloom into colonies that grow into the skins and fruits of oranges do they become visible.
Growers and shippers drench fruit but homemakers should fight the spread of the two common molds by cleaning the refrigerator with soda water -- a tbsp. of baking soda on a qt. of water -- followed by a rinse with 3 tsp. of bleach dissolved in a qt. of water. Discard paper towels or other porous drawer liners frequently and keep towels and wash cloths clean and dry. Discard fruit with visible blooms.