Now that I look at the photo, the spots appear more brown than yellow, but I was unable to edit the question. I could only modify the title. Looking forward to your answer!

Great question. 

Are you seeing any insect pests on the cucumbers?  I'm wondering what the white specks on the green leaves are from-- possibly squash bugs?  Are they producing cucumbers yet?   From the photo, it is hard to say exactly what is causing the brown/yellow spots, but it could be from a fungal disease such as downy mildew.    If the plants are producing, I would recommend removing the affected leaves and putting in the trash (do not compost or leave in the garden area).  You might consider using a fungicide as a protectant on the new leaves; however, if you are harvesting, be sure to follow label directions.  Be sure to water in the morning and try not to get water on the foliage to prevent fungal diseases. 

Consider replanting a new crop of cucumbers in case these die off early. 

Good luck-  Ashley