Good afternoon,

Thanks for reaching out. It looks like your cucumber may have an infestation of spider mites. I'll include a link here for more info. The leaf damage is on par for spider mites and the "dirt" on the underside of the leaves is a dead give away. You can try a few things to treat them. You can use a strong jet of water on the plant to remove/kill them. If you do this, repeat it a few times to make the biggest impact. They don't really react much to pesticides, but if you are desperate, the fact sheet I linked has a few good options to choose from. Proper watering during hot, dry periods is the key to preventing further damage and stopping a more widespread infestation.

Please reach out with any other questions,

Brody K

Colorado Master Gardener Apprentice