Daconil, contained in something like Ortho MAX Garden Disease Control, a concentrate which can be mixed with water and sprayed, will probably control it.

Your conditions look mighty damp. Just right for spreading fungus. Your situation would be much improved if the sun comes out I think. But a single application can have a dramatic effect in suppressing spread to new growth. It won't repair the damage already done.

I had that happen to my cucumbers last year...I thought it was the aphids you could see flying around everywhere.

I don't know what it is but I get that on my cucumbers every
year. As soon as I see it I cut the effected leaves off. It doesn't
stop it, but delays it, and I still get plenty of cucumbers. I just
started to see signs of early blight on two of my tomatoes. Took
all the lower branches effected off and sprayed with the Ortho
Max. You have to catch the disease early with ortho max. It may
or may not help.

I ended up taking a sample to my local garden center before I saw the replies here. They did indeed identify it as a fungus. I've removed the affected leaves and started using a neme leaf extract spray on the cucumbers and nearby tomatoes. No new spots since I started doing this Sunday morning (its now Tuesday morning).

Thanks all!