I have noticed some dead spots on my cucumber plants. It's as if the
vascular system is cutting off certain spots and letting the spots dry
out like leaves in the fall.

These images look similar, but they are more randomly placed and not
necessarily on the perimeter (as these images appear to me).

http://lynnsgardenjournal.wordpress....ay-bad-fungus/

Last year I did have a powdery mildew problem with cucumbers and
cantaloupes. This year I don't see any powdery mildew. I've also been
doing my best to water from below, avoiding wetting the leaves.

Any words of wit?

They are close to a pair of 40W fluorescent lights and the affected tray is positioned near the end of the fixture, possibly near the ballast. (I cannot know where the ballast sits without disassembling a metal housing encasing the fixture.) Damaged leaves are usually older--not necessarily the top-most and, hence, not always closest to the lights.
I water the soil from the top, trying not to wet the leaves. When I first noticed the condition the soil was somewhat on the dry side. These plants are otherwise ready for hardening off and will probably be root-bound by the time I set them out.