I made mounds for my zucchini and yellow squash, then planted rows of carrots in between them...just close enough for the carrots to be totally overhung and shaded by the squash as their leaves spread out.

All my squash eventually got covered with a whitish, mildew-like substance. After it really got out of control, I found Web sites describing the problem and recommending that you spray the leaves with one part milk to 10 parts water out of a common spray bottle. Next year, I'll be able to catch it as it begins and hopefully stop it.