And it is called powdery mildew:
Powdery mildew is a fungal disease which affects many types of plants. Cucurbits like zucchini, squash, pumpkins, melons and gourds are all susceptible to powdery mildew. It usually appears on leaves as small round white powdery spots which quickly spread and cover the entire leaf. In our garden here at the yardstead we've lost many zucchini, squash and pumpkin plants to powdery mildew. (source from Yardstead.com)
Yup, that would be us. It has affected the zukes, summer squash, butternut, gourds and pumpkins! AHHH and it gets worse:
It can be controlled with quite a bit of effort and regular applications of a fungicide. Once you get it in your EarthBox (garden bed) though, it will stay there. The next growing season you either need to plant veggies that are not susceptible to PM (i.e. tomatoes, peppers, eggplant) or you need to get rid of your potting mix and sterilize your box with a mild bleach solution. This would be why you get it two years in a row. (from Earthbox forum)
Double AHHHHHHHHHHHH Empty out all the soil, bleach and start all over..................... that sounds like way too much work! And would definitely increase the price per vegetable to way past ridiculous!
For now we are trying an organic (and cheap) solution of 9 parts water and one part milk to try and ward off this awful stuff. We'll see what happens next...................