Looks like cucumber mosaic virus. (Plus some slug damage.) But yellowing leaves can be many things, including lack of nutrients or too many, over/under watering, bad weather and soil conditions and so forth. If you poured a lot of dirt on top of the
root zone, the plants could be suffocating.
If the other plants in the area are doing well, and the plant doesn't show signs of stem damage at the soil line from slugs, I would suspect CMV and plant a resistant variety next year.