I'm new to this, but I'm having the exact same problem. It STARTED with the 2 small leafs at the bottom and once they were all the way yellow, the yellowing started on the next node up...Then those died as well. I hope this doesn't continue. From what I've gathered it could possibly be a nitrogen deficiency but like I said, I'm new and could be 100% wrong. Just picked up some 30-10-10 food though, gonna see if that helps.