Vaguely reminiscent of early stages of cucumber mosaic virus, but in that case the leaves would have yellowish round spots. I could imagine something like that in your picture. See https://www.growveg.com/plant-diseases/us-and-canada/cucumber-mosaic-virus/. I think what eventually happens is that the leaves get all crinkly, deformed, and brown. It is reported that, at least in lettuce, mosaic virus first affects older leaves. Your picture doesn't look particularly worrisome, though.

Thanks Dan. Yeah, I'm thinking I shouldn't get too worried about this yet. While I have you here, can I ask you this: Would it be good to keep the bottom of my cucumber vines (on the trellis arch), cleaned out of lower leaves? I didn't know if that would help with ventilation.......like it does with tomato plants? I do grow marigolds very close to them. Thanks for your input!

I only grow Armenian cukes. I don't do any leaf cleaning, really, though I pull off old dying leaves. If they are on a trellis, ventilation can't be much of an issue.

Vgkg....Nope, no cuc beetles. These are County Fair cucumbers and lack the bitter gene, which attracts the cuc beetles. They are the only variety that I can grow.

That's a fair point about cucurbitacin, which cuke beetles certainly like. But they'll go after things that don't contain it. Cuke beetles will attack corn, tomatoes, as well as flowers on fruit trees.

Hey Dan, you mentioned you pull off dying leaves. Should I cut the stem off on those dying leaves back to the main stem? I didn't want to do anything that would cause problems. I really think the seeds I got last year weren't really County Fair. Last year I had tons of bugs and lots of big stems died. This year, I ordered from a different company, and they are doing phenomenally. I haven't seen "bad" bugs at all. But the lower leaves are looking like the ones in the pic I posted. I just didn't want to go overboard cutting things back. I know the climate is different than last year......so who knows what's going on. I have seen VERY few bugs of any kind this year (including butterflies, unfortunately). Anyhow, I'm hauling in tons of cucs every day. Pretty crazy, but I love it! Thanks for your feedback about cutting back..

I just pull the dying leaves. Mainly just to clean up the plant, and allow better sunlight penetration. Of course, lower leaves will be the ones that get less sun, and may stay wet longer, which is conducive to fungal infections.