That looks a lot like frost damage. It looks like the damage is only on the older leaves and the new leaves are OK, is that correct? Any chance this plant has been exposed to cold weather? If that's what happened, the plant will probably grow out of it. Once the new leaves have grown large enough, just prune off the frostbitten leaves.
Discussion
I think that was right. Thank you so much for the answer! I just didn't want it to be a disease and have it start affecting the other ones. We had very late frost that knocked out half my cucumbers :( everything else was okay after a couple weeks recovery)