i've had this kind of problem many times before. it seems to be a normal process, when a leave no longer become efficient at producing energy, the plant drops. I have read other documents though that say it is a trace element deficiency, especially if it is isolated to a specific area, like the middle of the plant only.
I have that info on the type of deficiency it may be but i will have to look for it, so let me know if you would despiratly need this info.