Twig die back in citrus plants is a common symptom of zinc deficiency. It can be corrected using foliar sprays.
Limb die back can be a symptom of copper deficiency - particularly in young citrus. Gumming can also occur. See more on Citrus Gumming.
Growers should check using tissue analysis and apply the appropriate micronutrient fertilizer to the leaf to ensure supplies of the deficient nutrient are not limiting growth.