Die back of citrus is caused due to the deficiency of ..........................

Explanation

**Step-by-Step Solution:**

1. **Identify the Condition**: The question asks about the cause of "die back of citrus." This is a specific condition that affects citrus plants.

2. **Research the Deficiency**: To answer the question, we need to know what nutrient deficiency is linked to this condition.

3. **Determine the Nutrient**: After researching, it is found that die back of citrus is primarily caused by the deficiency of copper.

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