First guess is that it's either Calcium or Boron deficiency (these deficiencies tend to start at the top)

check this link for troubleshooting
http://www.icmag.com/ic/showthread.php?t=11688

Remember - don't try to correct a deficiency by adding more of the element at first...most deficiencies are caused by an improper pH. Without correcting pH first, adding ferts will only make things worse.

good luck