I had a problem like that about a year ago. (hydroponics). At any rate, the leaves started yellowing and falling off from the bottom up until the whole plant died. After 3 or 4 plants were lost and I was still clueless, I talked to someone at my grow shop. He said I probably have a virus. He told me to check my roots and see if they looked healthy (crispy & white), or sick (brown & dungey and maybe even slimey). He told me to add 1 tsp per gal of nutrients of hydrogen peroxide. (the kind you buy at the pharmacy store). That was the end of my problem.

wallhofen