I posted this question to eXtension's Ask an Expert. https://ask.extension.org/questions/3...
This is what I got
"There are several common viruses that can affect cucurbits, including Cucumber mosaic virus (CMV) and Watermelon mosaic virus (WMV). Infected plants may be stunted or have leaves that are mottled, crinkled, or a light green color. However, your leaves really look fine except for the yellow areas.
This can also be caused by a nitrogen deficiency. If you Google "leaf yellow from lack of enough nitrogen" and go to images, you will see similar leaves. The only way to know for sure is to send a sample to a plant clinic. However, since the picture is a re-post, I'm not sure how you can verify it unless the person who posted the picture has samples to send.
I would also recommend a soil test. It's especially important to do this every year or two because vegetables are heavy feeders. Nitrogen is the most volatile nutrient, easily lost from the soil by gasification or dissolving in rain water. However, soil rich in organic matter holds nitrogen better, and a bit of composted manure can re-establish the nitrogen in the soil.
I hope I've answered your question. Please feel free to write again if you still have questions.