Mouldy apple. Food turns mouldy when contaminated by a fungus. Microscopic fungal spores have landed on this apple, forming a fungal body. The fungus has secreted digestive enzymes, breaking down the apple's tissues to obtain nutrients for the growth of the fungus. This decay turns the apple brown. The advanced state of decay of this apple has led to parts of the fungal body (white) protruding out of the apple. These parts will contain structures to form and release reproductive spores to spread the fungus to another apple or suitable source of food. Decay is most rapid following a break in an apple's skin, the probable explanation for why the mouldy apple decayed before the others.