Have you used a copper fungicide? If you think you have powdery mildew it is supposed to work to prevent as well as stop it. I always use my bleach solution when I get powdery mildew and it is very effective; but any affected leaves will die and dry up within a few days. I have used it successfully on squash, melons, tomatoes, and cucumbers for getting rid of powdery mildew. The bleach spray has worked for me to kill or stop any kind of fungal disease but it has no preventative use so a fungicide would be useful afterwards to prevent further outbreaks. If it is not fungal or bacterial and is some kind of systemic disease like fusarium then the bleach spray will have no affect.
Did you check for spider mites? They will cause yellowing and can really sneak up on you because they are so hard to see.
I hope you can find out for sure what you are dealing with. Right now you remind me of when years ago I first encountered TSWV. I tried everything in the book and watched in frustration as one plant after another died from it before I knew what it was. Hopefully your problem is more amenable to a solution.
Bill
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