Citrus exanthema is caused by

Citrus exanthema is caused by a deficiency of Copper. The disease is usually attributed to malnutrition and is not infectious.
The dropping, blemishing, and cracking of fruit form one of the most common symptoms of exanthema disease in fruit. Brown glossy areas occur particularly on side of the fruit. The surface shows small dots of gum. Cracking is common. The fruit is undersized and often a pale yellow.
It can be treated with a copper treatment.