It is hard to say exactly but you may be over watering. If the tree is potted in a pot and not a garden or orchard, make sure the pot has good drainage. The roots should not be in standing water.
You can pick up a meter at your garden store to tell you if the soil is to wet or dry. You might try to find something like that.
Again, if the tree is in a pot you might want to try repotting it in a larger container. If to much water is the problem, putting some rocks at the bottom of the new pot and using a good potting soil should help.
The other possibility is some type of parasite infestation. You should be able to see some types of parasite infestations. Check the green parts for small dots. Not sure if these are your problems but it would be my best guess.