I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
There are several things that can cause yellowing of a cucumber leaf. It the yellow is in spots or patches it is most likely a fungal disease. Your local garden center can suggest an appropriate fungicide if you wish to treat the plants. With the season being so far along it may be better to just live with the situation as those plants are going to be really struggling to set and mature fruit with the heat and then replant in late July or early August for a fall crop.
If the yellowing is primarily on the older leaves try fertilizing with a little nitrogen.
Thanks for the question. Please stop in again soon!
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