is this some sort of burn? or an infection?
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It looks to me like a cucumber plant that is just about "pooped". In our region, the same cucumber plants don't continue to thrive and produce the entire summer. I plant a second crop about 4-6 weeks after the first crop, and a third one 4-6 weeks after the second. When the leaves start looking like this, typically, insect pests move in quickly and then you have double trouble. Pull out the plants and plant over a longer season next year.