Hello. We have zucchini, butternut squash and cucumbers with powdery mildew. We have snipped the affected leaves and sprayed a mixture of baking soda, mild dish detergent and canola oil in a gallon of water. A recipe we got from the internet. We do not know if that's the best organic solution. Thank you so much!
Due to weather conditions this year powdery mildew has been showing up in gardens. It usually shows up later in the season as plants are maturing.
It is recommended to select resistant varieties to powdery mildew. See the list from Cornell http://vegetablemdonline.ppath.cornell.edu/Tables/TableList.htm
Homemade solutions are generally not recommended as they can be phytotoxic to the foliage and cause more damage. There are horticultural oil products labeled for powdery mildew on cucurbits. However, they have to be sprayed as soon as you notice symptoms and cannot be sprayed in high temperatures as this can damage the foliage too. Follow label directions.
Here is our page on powdery mildew http://extension.umd.edu/hgic/powdery-mildew-vegetables