Any weedkillers any where in the vicinity?

No weedkillers. Would too much sunlight cause this? I know they get a solid 8 hours if not more.

Could be Curly Top Virus (CTV). If it is, the plant will continue to decline but not die. The disease is transmitted by the Beet Leafhopper. Control is useless
because it has infected the plant with the virus and it long gone. To prevent CTV cover the plants with 'row cover cloth' or Rami, which can be purchased a fabric stores and is a lot cheaper, from planting time until late May/early June. If the plant continues to deteriorate, discard it the garbage not a compost pile or the like.

Any neighbors use weed killers?

I ask because the stuff can move on the breeze. Every year, my neighbors damage my plants with their weed killers.