Question

Die back of citrus is due to deficiency of:

• A. Iron
• B. Manganese
• C. Copper
• D. Boron

Hint:

Associate the following diagnostic deficiencies in Citrus:
Die-back Exanthema $\rightarrow$ Copper ($\text{Cu}$)
Interveinal chlorosis $\rightarrow$ Iron ($\text{Fe}$)
Lori Hard fruit $\rightarrow$ Boron ($\text{B}$)

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Plant nutrients are classified into macronutrients and micronutrients based on the quantities required by plants for healthy growth and development.
Micronutrients, though required in trace amounts, play critical roles as cofactors for enzymes, catalysts in redox reactions, and structural components of vital macromolecules.
A deficiency in any essential micronutrient leads to specific, diagnostic physiological disorders and visual symptoms in crops.
In citrus cultivation, nutrient deficiencies manifest as distinct vegetative and fruit disorders that severely impact orchard productivity.

Step 2: Detailed Explanation:
Let us analyze the deficiency symptoms of the listed micronutrients in citrus crops:
1. Copper (Cu) Deficiency:
Copper is an essential component of several enzymes involved in photosynthesis, respiration, and lignin synthesis, such as plastocyanin, cytochrome c oxidase, and polyphenol oxidase.
In citrus, copper deficiency leads to a well-known physiological disorder called “Die back” (also known as exanthema or red rust).
The initial symptoms appear on young shoots, which develop unusually large, dark green leaves on vigorous, s-shaped twigs.
As the deficiency progresses, pockets of gum form under the bark of young twigs (gum pocket stage).
Eventually, the twigs begin to die back from the tips downward (die back stage), and the bark may split, exuding a reddish-brown gum.
The fruit may also display brown, gummy spots on the rind, a condition known as “ammoniation.”
Therefore, copper is the nutrient responsible for this specific disorder.
2. Iron (Fe) Deficiency:
Iron deficiency causes interveinal chlorosis, primarily on young leaves, where the leaves turn pale yellow or white while the veins remain distinct and green, but does not cause the characteristic die-back twigs.
3. Manganese (Mn) Deficiency:
Manganese deficiency leads to mottle leaf or interveinal chlorosis, appearing as light-green bands between the main veins on young leaves, similar to mild zinc deficiency.
4. Boron (B) Deficiency:
Boron deficiency causes hard, misshapen fruit with brown, gummy spots in the albedo, corky veins on leaves, and death of the growing terminal bud, but is not the primary cause of the classic citrus die-back.

Step 3: Final Answer:
Consequently, the micronutrient whose deficiency causes the classic “die back” disorder in citrus is Copper.