I have a couple of cucumber plants that I set out on May 11 (Diva and Tasty...
This looks to be either environmental damage (wind etc.) or cucumber beetle or a little of both. It does not look serious and no treatment is needed- you can just leave it alone. The smaller leaf may brown out and then can be removed.
You don't think it could be mosaic?
Our fruit and vegetable specialist looked at your photo earlier and did not mention that as a possibility. He is out of the office now, but we generally think that if it were mosaic the whole plant would look ill when it actually looks pretty good.
If it gets worse this weekend, please take another photo or two and attach them to this reply and we'll have him look again.
If it were mosaic, which is a virus, there is no treatment and the plants would be removed and destroyed.