White Spots on Zucchini Leaves

Look at this website:
http://www.pioneerthinking.com/tv-mildew.html

It describes successful research on treating mildew on zucchini and other squashes by spraying with diluted milk! This looks easy and I am trying it tomorrow!

Sounds like Powdery Mildew. You can google 'powdery mildew' and you should be able to find some photos to compare to what you have. This is common in all the cucurbits - cucumbers, squash and zucchini. I use a fungicide spray. Get one that specifically lists powdery mildew. Spray tops and bottoms of leaves. Untreated, you will still make some zucchini, but the yield will be far less than normal and your plants will die rather quickly. I normally start treating as a preventive when my plants (from seed) are big enough to start fertilizing. Usually once a week for 3-4 weeks, until my plants are too large to bother trying to spray. That has worked well for me this year. Last year I got the powdery mildew and only got 6 pounds total off 6 plants. Very sad.

Go to the Google search engine and look for gardening websites. I believe you will find your answer on one of the websites.