skippity, there is nothing to worry about. The yellowish leaves are just old leaves that have outlived their lives. They are turning yellow because the tree is removing nutrients from the leaves to reuse in other sections of the tree, nothing is wasted, then the leaves will be discarded. A citrus leaf has a life span of approximately 18 months. When a leaf can no longer fulfil all that is required of it, it is quickly discarded. There is no welfare plan for old citrus leaves.