Chandramohan-
Hmm that sounds odd. Personally I'm not sure what to tell you - I've never had that happen EXCEPT - when the trees are beginning to go dormant for the winter, the terminal leaf bud will kind of turn black and appear dead, but it's fine. If this is happening in the middle of the growing season, however, I'm not sure what could be wrong.
What are the growing conditions: temperature, watering/fertilizer regimen, type of pots and growing medium (or in the ground?)?
Pictures might be helpful too.