Bennion I suspect that your tree is pot bound and that it has used all the available nutriments in the soil in the pot. At 6 years old it probably needs to be planted in the garden, failing which in a much larger container with fresh compost and lots of feeding. Snag is it is now late in the year to plant/repot but I think you are going to have to, a tree without leaves will not survive.

i agree just be real carefull when you repot it so you disturb it as little as possible and then keep it well waterd.