Looks to me like fusarium wilt. This is a fungus that is soil borne. It may have come from the soil the plant was grown in or from your own garden soil. There is no cure and it is best to rouge out the plant and destroy it. Prevention is the key. Proper soil pH, consistent watering, avoid water splashing soil onto the plant. A good mulching program is the best growing method. A layer of mulch between the soil and the plant leaves helps. My personal choice is layers of newspaper on the soil covered by 6-8 inches of straw on top. There are many other mulching programs.
You plant will die quickly if it is indeed fusarium wilt.