Looks like powdery mildew. Some say diluted milk will do it, also baking soda solution (I don't measure). Soak both sides of the leaves. Then do it again every day until you see new leaves, then I'd clip off these sick leaves. This spreads with gloves and tools, so be careful. Soak the new leaves also. P.M. is a fungus that supposedly grows in cool, damp weather, though it grows in hot, humid weather too in my experience. I spread my plants at least 3 ft apart, so they get lots of good circulation. Squash is very vulnerable to this.

Thank you.

If that doesn't work, you could look into Serenade or Actinovate. Both are bacteria based fungicides and work for a variety of common plant problems. They are organic fungicides and I've been pretty impressed with their success.

That is NOT powdery mildew. That is the normal mottling of zucchini leaves. The reason the one leaf is yellowing is because the plants are stressed. Plants of that size need to be in the ground and not in pots. If I ever buy zucchini transplants I only get the ones that have just their seed leaves showing. Cutting off a leave on a plant that small will only further stress it and slow its growth. This is what powdery mildew looks like:

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Notice that they are initially round patches of fungus that will eventually form a covering over the whole leaf. You can tell yours is not powdery mildew because the coloration occurs only in the region where two leaf veins come together and coloration is not circular.

Thank you Weirdtrev! I'm glad to know this.
Good stuff.. : )

It's good to know that it's normal. I can see the same greying on some of the vein branches on my zucchini: