Thanks for the deep dive. I definitely didn't look closely enough to know whether I had downy mildew or mosaic virus - from pictures online it looks like a real possibility.

I had been trying to stick with heirlooms in the hope of saving seeds, but saving cucumber seeds is such a PITA anyway (need to bag male and female flowers and hand pollinate) that I'm OK going to hybrids. I'll look into those you mentioned.

Thanks again.