Exanthema in citrus is caused due to deficiency of

Zinc deficiency causes exanthema (also known as mottle leaf) in citrus plants. Symptoms include yellowish mottling between leaf veins, small narrow leaves, stunted growth, and reduced fruit size. Zinc is essential for several enzyme systems including carbonic anhydrase and alcohol dehydrogenase. Manganese deficiency causes different symptoms (interveinal chlorosis), boron deficiency causes corking and cracking, and copper deficiency causes wilting and dieback.