Which of the following nutrient elements is responsible for the dieback of citrus?

The decline in the yield of citrus fruits is commonly known as ‘dieback’ or chlorosis.
It is a syndrome expressing many disorders in the plant that leads to a decline in productivity, reduced productive life and poor fruit quality.
The deficiency of copper in the soil is responsible for the cause of the 'dieback’ of citrus fruits. It leads to poor growth, delayed flowering and plant sterility in citrus.