In your picture it looked very clear that at least one leaf had curled upwards at the tip. Is that the normal curl you are getting? If so the following might be your problem.
 
Mg - MAGNESIUM (Mg)
Lower leaves yellow and may even turn white while veins remain dark green. Blades die and curl upward.
The one thing that makes me question if that might be correct is it did not appear that your plant's veins have remained dark green. Possibly that is due to an advanced state and it could mean it is a totally different problem or several different problems combined.
Not to be an alarmist but I tend to believe that you may have more than one issue to deal with but I am not sure what else to add to the above. A few more pictures, hopefully of decent to high quality and a very detailed description of how things began and how they progressed would help people to better be able to assist you.