I am having the same problem. It's on my summer squash and zucchini plants. Hope someone responds with knowledge!

Yup, that is powdery mildew. What to do is as follows:
Remove all infected leaves and discard away from plants. Spray with fungicide to retard further growth and spread of the mildew. Continue this process throughout the rest of the plants life. Spraying will have to be repeated after any heavy rain. Prevention is key.

Commercial fungicides are available at the big box stores, just look for one that addresses powdery mildew. I'm trying a Copper based fungicide this year.

For organic measures, there are posts on this site regarding the use of milk based solutions or baking soda based solutions. I tried these last year, and they helped some, but since I'm growing a lot of squash this year, I'm breaking out the bigger guns.

You can search the site for milk or baking soda to find proportions or google powdery mildew and milk or baking soda on the internet.

Good Luck!
Susan

I'm having the same problem with my squash, gourds, and cucumbers. This is my first experience with mildew, so I'm learning as I go. I've been cutting off the leaves that are badly infected and spraying the rest with Organocide. It's an organic mix of oils that I got it at Home Depot. It seems to stop the spread for about a week, then I have to spray again. The plants seem to tolerate it pretty well so far, but I have noticed that my cucumber leaves are speckled. I'm thinking it is probably from the Organocide. However, they are still growing and producing.