I need a few more clues to solve this mystery. What type of tree? Is it yellow from the tips inward or mainly from the interior? Are the leaves dropping? Trees will turn yellow from too much water and poor drainage, lack of moisture, sunscald, and from soil borne diseases. If you send a photo I may be of more help. Do not give up. Sometimes trees that struggle when young outgrow their issues as they mature. Keep growing, Marianne Binetti