Dead spots on cucumber plant leaves?

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). /
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?
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