The leaf curl is most likely caused by stress on the plant. I'd guess a perched water table is the reason, due to the ratios of the growing medium recipe over at the GB kid's website....

EG

Thanks for the reply. While I built the buckets using their videos, I did not use their growing medium recipe. I used the 3-2-1 soil mix recommended by Raybo in his Earthtainer pdf.

3 Parts Potting Mix
2 Parts Pine Bark Fines
1 Part Perlite

The other two plants in the same bucket are doing great. So it's really odd to me.

hmm....has your area received alot of rain lately? A torrential downpour can quickly overtake moisture control conditions when the top of your container isn't covered by something to shed the rain off.

EG

No rain at all.

I have been keeping an eye on it and while the new leaves that are forming look the worse, it is still growing and the leaves that were almost black from curl, have started opening and turning green.

wayner123, Three plants of the same variety, are grown in a homogenous mixture and under exactly the same cultural practices.
One plant exhibits an abnormal symptom at the growing area.
The first reaction is that there is something going on in the roots and or conducting tissues. If this were true, all three would be affected.
Therefore it must be something external and affecting only the one plant. An insect comes to mind. Aphids? Thrips? Leaf roller of some sort? Stem borer?
The implication here is that this is the first visible sign of what is likely to be a more widespread presence of the offending (offensive?) organism.
A minute examination of the other plants is indicated; a seek-and-destroy mission - if you will pardon the expression.
There is the remote possibility that the roots of that particular plant encountered a patch of "salts" and cause a "chemical" burn. This can be considered in context of your fertilizing practices.