Oranges and other citrus fruit often develop mould growth after harvest, which is caused by green mould fungi or blue mould fungi. These types of mould produce fungal spores on the surface of oranges that are airborne and can infect other fruit and even storage rooms fruit is kept in.
Mold growth is caused by fungal spores that are carried on the air to infect other fruit in close proximity. Green mould is caused by the fungus Penicillium digitatum, and blue mould is caused by Penicillium italicum. These types of fungus survive in the soil and germinate to infect fruit. Both types of mould are suppressed when the temperature falls below 4.44 degrees Celsius. However, when the temperature warms, oranges can become infected and completely covered in mould in a matter of days.
Green mould is a powdery substance that can occur while the fruit is still on the tree or after harvest. The early symptom of green mould is a white, mouldy area on the surface of the orange. As the area becomes larger, green mould spores are produced, and the mould looks green instead of white. Oranges with green mould decay and often shrink. Blue mould symptoms are similar to green mould's, except the mould on the orange is blue instead of green. Fruit with blue mould also shrinks and becomes soft and decayed.
When oranges are harvested, care should be taken to avoid injuring fruit, which gives mould fungi an easy point of entry. Because mould spores are airborne, infected fruit should be discarded to avoid infecting other fruit. Keeping oranges cool during packing and shipping can help reduce the damage from green or blue mould.
Green and blue mould are resistant to many chemicals such as fungicides after harvest. Proper sanitation of packing houses and storage facilities can help reduce green and blue mould development in oranges. All soak tanks in packing houses should be sanitised to avoid the spread of mould to other fruit.