Hail can cause leaf damage. If your area had hail recently you may want to consider this. Anthracnose, leaf blight and leaf spot are three diseases that could cause the discoloration and death of tissue shown in your photo. I am including a link that shows photos and information about these diseases. I would encourage you to take a look and see if the any of these match what you are seeing. From the photos it appears leaf spot may be the challenge. If the plant continues to decline I would suggest starting over, including a new space with new soil if in a container. 

https://apps.extension.umn.edu/garden/diagnose/plant/vegetable/cucumber/leavesspots.html