Which mineral deficiency causes the 'Exanthema' disease in citrus plants?

In plants,the root system is well-developed due to:

It is observed that the deficiency of a particular element shows its symptoms initially in older leaves and then in younger leaves. Name two elements that are highly mobile and two that are relatively immobile.

Name the deficiency symptoms shown by plants and their causative elements.

If a plant shows a symptom which could develop due to deficiency of more than one nutrient,how would you find out experimentally,the real deficient mineral element?

Deficiency symptoms of nitrogen and potassium are visible first in