Thank you for your question.  Spots such as these are typically caused by sucking insects which live on the underside of the leaves.  Could you please check to see if there are any there?  They may be very small.  Thanks!

Thanks, Jessica!  I think your plant has a virus called cucumber mosaic virus, which is carried to the plants by aphids, which may have been there but are now either gone or too small to be seen.  I don't see the downward turning leaves, but it may be on other ones not in the photo.  This is an article with photos of them, with control recommendations:  Cucumber-Aphid | Pacific Northwest Pest Management Handbooks (pnwhandbooks.org) Here is an OSU publication on the virus disease:  Cucumber (Cucumis sativus)-Virus Diseases | Pacific Northwest Pest Management Handbooks (pnwhandbooks.org)

I suggest you heed the article's advice, and remove any leaves with this yellowing, and destroy them.  Check your plants often.  Neem oil only works if sprayed directly on the insect, which feeds on the underside of the leaf.  A strong stream of water blasting them off works just as well.  Hope this is helpful.  Good luck!

Could you please send a closeup photo of the plum leaf?  There are viruses that attack them, but rust, a fungal infection, is a look alike, and needs different controls.  Thanks!