Early blight?
Not sure, but mine did something quite similar.

Looks like something is chewing on the leaves, and possibly entering a virus.

Have you done anything to deter squash vine borers? I think they also go after cukes.

My DH has been religiously placing diatomacious earth (for gardeners--the pool kind can cause breathing problems) around his vines after each rain, and it seems to help.

Some place floating covers over their squashes to deter borers--you might want to read some of the old threads on squashes and cukes.

Have you seen any cucumber beetles around? Yellow with black stripes (north) or spots (south).

Mine did the same thing. I have had a lot of cucumber beetles around, so I am sure I lost mine to bacterial wilt. I knew the darned beetles were around and did everything I could but put out poison and they still got all but one of my cukes.

Thanks. I haven't noticed any cuke beetles, but I may need to look harder. Are they fairly obvious, or do they hide pretty well? I don't think it's vine borers. I'm very familiar with them as they destroyed my squash for a few years before I figured them out. Now I use permethrin to somewhat keep them under control (is there an effective non-chemical control for vine borers?). I've been dusting the cukes occasionally too. They've still gotten to a few squash vines, but the cuke vines are much smaller in diameter and very easy to see. I don't see any physical damage to the vines themselves.

Thanks,
Phillip

Cuke beetles like to hide in the soft and tender spots, namely the new growth, the flowers, etc. Look for them there. Once you find them...well, good luck, it ain't easy to get rid of them.

This is my first year gardening, but I've had quite a problem with cuke beetles...on everything, including my non-veggie flowers. Also, I've noticed they are quite intelligent. It's like they see me coming and try to hide on the backside of leaves when I approach or climb down the plant to where I have trouble seeing. I even had one that I knocked onto the ground of my garden, and when I went to look for it (to kill it), I couldn't find it. So I sat and waited. The darn thing had grabbed a small piece of mulch and flipped over with the mulch on top of it (as camo)! As soon as it made a run for it, I tried to get it and it did the mulch-camo thing again! Crazy. But I eventually caught that one and he met his maker. Now I've got the second generation on my beans to keep me busy. Somehow my cukes are still alive, but I'm sure it's just a matter of time.