Citrus will often drop fruit when it is under drought stress or carrying too large of a fruit load. Try to maintain plenty of soil moisture without keeping it too soggy wet which is also very detrimental. The tree will need to get plenty of light also so if a larger tree is overshadowing it some pruning of the larger tree may be needed. Don't fertilize the tree too much especially with nitrogen. A little goes a long way an too much results in vegetative growth at the expense of fruit production.
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