When the leaves of a citrus tree turn yellow, the tree usually has a mineral deficiency or is diseased. Mineral deficiencies typically are soil related and can be fixed by adjusting the pH. Over watering can lead to diseases which cause yellow leaves and rotting. Excess sunlight also can cause yellowing and curling of citrus leaves.
Citrus trees are prone to several diseases, all of which can cause yellow leaves. Citrus greening is one of the more common diseases seen in citrus. An insect, the Asian citrus psyllid, carries this infectious disease and transmits it while feeding on the tree's leaves. Infected trees infected bear leaves that are discolored and fruit that is misshapen and bitter. There is no cure and many infected trees die within a few years.
Citrus canker disease can be identified through sores found on leaves. This bacterial disease is highly contagious and rapidly destroys the health of citrus trees. Fruit production will cease when the tree becomes infected.
Sweet orange scab is a fungal infection that causes fruit to drop prematurely. The symptoms include legions appearing on the fruit's rind and throughout the leaves. While this infection is unsightly, it does not affect the taste of the fruit.