anyone had this problem with cucumber grown from seed,grown them for years and never had this problem,the leaves have a mosaic appearance on leaves then they become floppy..
I think this may be due to water lying around the base of the plant in the early stages.
The stems which are delicate and vulnerable to cold (especially during the night) should be planted on a mound of soil to avoid water accumulating around the plant.
Agree with David K. about mounding up compost so stems don't rot.
Water little and when needed and don't soak them. Don't expose to direct sunlight. Wonder if you've recently re- potted them. Sometimes leaves do look a bit as you describe but don't panic about mosaic disease.
Alternatively instead of planting them on a mound, plant them in a flimsy pot, 3' or 4' is sufficient and cut the bottom off, then plant them out with the pot. When you come to water do this outside the pot. The stem doesn't get wet and water is drawn up by the roots.