If you don't see any obvious pests on the plant, it is probably a disease, like fusarium wilt (a fungal disease). Best to destroy the plant and try not to pass on the disease to other plants nearby.

Since it appears you are growing these plants in pots, I wonder if poor drainage could be a cause. I try to avoid using anything but the simple black plastic pots that plants are grown in at the nursery -- these are the only ones that have large, well-located drainage holes (unlike the terracotta pots that only have a hole or holes in the flat of the bottom). I also avoid anything sold as a potting mix, unless I add a bunch of cinder or other well-drained material to it.