As they said, you have to give it time. And while there can be other reasons for lemon leaves to turn yellow, one cause can be simply a sudden change in nutrient profile, such as planting out. I suspect that one, because it's rather sudden for a newly introduced plant to acquire disease. And your tree was almost certainly grafted onto deciduous stock, so it will go through seasonal changes in fall. It takes up less nutrient and can show it. And that might be compounded by the first suggestion.
