Several factors---individually or collectively---may cause the leaves on an orange tree to turn yellow. Accurate diagnosis of the reason for leaf yellowing requires close attention to other parts of the tree, its environment and familiarity with the care it receives.
Diseases such as foot rot, root rot, alternaria brown spot, citrus scab, greasy spot and phytophthora; a nitrogen, magnesium, iron or zinc deficiency; damage caused by ants or maintenance equipment; or cold winter temperatures combined with improper watering practices may result in yellow of orange tree leaves.
Yellow leaves can manifest themselves in a variety of ways. The pattern of yellowing may point to the problem. For example, greasy spot causes yellow spots, but a nutrient deficiency will result in overall leaf yellowing. The University of Florida IFAS Extension website provides helpful photos of a variety of conditions that cause leaf yellowing.
Proper gardening practices---including irrigation---reduce the chances of diseases that lead to leaf yellowing. Other measures include the application of disease-specific fungicide and/or insect-specific pesticide, and corrective fertilisers.