It is hard to say much without seeing a picture. But it does sound like they are getting powdery mildew. Do you live in a very humid climate? I know that there are fungicides which can be applied. But I do not know how far gone the plants can be and still be saved. You might check at a local garden center, or better yet, your county extension agent.

I have seen the yellowing of older leaves, as you describe. If I recall, when I saw it, the plants were growing in a fair amount of shade. Is this the case in your garden?

Sure hope you get a harvest!

Here, we have had a rough year with pests, extreme heat, and at times, extreme humidity. Every squash I planted, with the exception of those from the c. moschata species, has died without producing.

George
Tahlequah, OK