Insects spread a STD. Examine for frass, they leave evidence. Usually its a systemic mosaic virus that is hard to fight once started. However....The stems look quite healthy from what little I can see so it may be an external attack, which might be fungal looking at the border of the dead leaf area. Its just too far away to tell much.

Yep all sorts of possible explanations ranging from weather, water, plant age to pests to any of 5-6 common cuke diseases.

Dave

Its just depressing. Cucs are as bad as tomato in catching diseases.