The symptoms--yellow, dying lower leaves, wilting, leaf curl, but continued slow growth--seem to fit "nematodes". Therefore I exhumed a plant to examine the roots. You can see the evidence I discovered in the link below.

At five inches under the soil, a stratum of white, perhaps mold, appeared in the sterile compost bed. However, it didn't seem significant. The roots appear normal and show no signs of gall or knotting, which are apparently characteristics of nematode damage.

The level of moisture in the soil seemed fine and evenly distributed. There was no pooling of water or rot around the tap root. Since it is hard to tell whether you;re watering too much or too little, this was interesting. I water once a week or more. It is tempting to water often because of the appearance of wilting, but I didn;t; and the soil one foot deep was still damp. Therefore I eliminate over-watering as cause of the problem.

I also eliminate nematodes, since the roots system seems healthy (anybody disagree?).

Hmmm. There's an insect on close examination of the plant. Is that a thrip?

As I said in a previous post, I believe I have slowed the progress of whatever this disease is by weekly heavy application of neem oil, an insecticide (and by one application of Diazinon).

I have now ruled out nematodes. Any theories or corrections appreciated.

Here is a link that might be useful: Roots 8.14.09