This is a fun plant to grow but exacting in its needs when kept as a houseplant. Your mango tree is native to tropical and sub-tropical lowlands of India and Southeast Asia. It is acclimated to warmth, humidity and bright light.

It is possible that the grow lights are positioned too close to the leaves and are scorching them or that the plant is receiving too little light. Be sure that the light system you have is correctly sized for the plant and is delivering enough light for enough hours. Artificial light is much weaker than natural sunlight and a bright-light, tropical plant may need 14 hours or more of strong artificial light, in the correct color ranges, depending upon how much natural light it is also receiving. 

Low humidity will also cause leaves to brown and drop. Use a pebble tray to keep air moisture above 50%. A simple hygrometer will help you check on the water vapor in the air around your plant.

Temperature should stay solidly above 60 degrees and warmer is better. Water thoroughly until water comes from the bottom of the pot, empty run-off and water again when the soil looks dry. Feed with a balanced, organic plant food every two weeks.

Good luck!

Courtesy of NYBG Plant Information Service