Question

Exanthema in citrus is caused due to deficiency of
(A)Cu
(B)Zn
(C)Fe
(D)Mn

Answer 1

Hint:It also forms a component of enzymes, such as phenolases and tyrosinase. It plays an important role in plant respiration.

Complete answer
For plants, copper is a critical micronutrient. If Cu deficiency is associated with either Zn or Mg deficiency, the symptoms of leaf and twig cannot be seen due to reduced development but the normal symptoms of fruit may still be present. Fruit signs are also the most accurate predictor for normal monitoring. Copper deficiency is responsible for exanthema in citrus. Fruits reveal patches of gum around the central pith. Exudation of the gum can also be observed on the rind.
The signs of Zn-deficiency are characterised by circular green bands on a backdrop of light yellow to near white around the mid-rib and main vein.
Iron deficiency causes interveinal white chlorosis first on young plants. The interveinal areas turn gradually yellow in extreme situations, with the entire region ultimately becoming ivory in colour.
Manganese deficiency leads to chlorosis. Chlorosis appears first on younger leaves, then spreads gradually to older leaves.

Additional Information:
Copper deficiency can cause "dieback", "ammoniation", and "exanthema" in citrus. These names are taken from the twigs' dying back, constant contact with strong application of ammonia, and excrescence on the twigs and fruit surface. For years, excessive applications of nitrogen fertilisers have been regarded as a leading cause of this issue, giving rise to the word "ammoniation".
Symptoms of Copper deficiency are:
1. The effect of copper deficiency is dark green leaves.
2. It causes exanthema
3. Reclamation disease (apical necrosis of young leaves spreading along the margins towards base) is caused in plants growing on newly reclaimed soil, where seed development is disturbed.

So, the correct answer is Copper (Cu).

Note:Copper as a component of plastocyanin is involved in photophosphorylation. Copper maintains nitrogen-carbohydrate balance.