Answer: There are a few possiblities here: if the leaves curl downward and drop, it could indicate a viral disease; if the leaves curl in a distorted way, aphids could have been (or are) feeding on the undersides of the leaves - doublecheck to see if you find aphids; if the leaves turn yellow as they are curling, it's probably a nutrient deficiency. Since your plants are in pots, you might want to move the affected plant away from the others until you can isolate the cause. I'd be inclined to prune off the affected plant parts to see if the new growth develops normally. If not, you may want to replace the plant rather than having to keep treating it to keep it healthy.
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