Hi there,

Are the cucumbers being grown in raised beds or directly in the ground? And are they currently planted in the same spot as they were last year, or in a different part of the garden?

They are planted in the ground in the same spot as last year. They are growing on a trellis. 

Here is a link to an informational sheet regarding the common problems of cucumbers.

https://extension.umd.edu/resource/key-common-problems-cucumbers/

If you scroll down to the virus diseases section, you will see leaves that show mottling like yours appear to have. This is generally indicative of a mosaic virus, which causes a yellow or green mosaic pattern or mottling of the leaves, among other symptoms.

https://extension.sdstate.edu/understanding-and-managing-cucumber-mosaic-virus

It is difficult to pinpoint exactly which virus it may be - and unfortunately there is no cure for it. The virus can overwinter in perennial crops and weeds, so it is important to practice crop rotation - I know this can be challenging for small gardens- and not grow the same plants or plants in the same family in the same spot every year. With any plants that appear diseased, I don't compost them after I pull them, but I am lucky to have a location where I can burn them. General garden sanitation is important too - keeping weeds that might already be infected at a minimum.

If you want to be more sure of what is going on, the MSU plant and pest diagnostic lab is available - for more info, click below. 

https://www.canr.msu.edu/pestid/

Good luck and thank you for using Ask Extension.