This year and last I have struggled with powdery mildew on all the squashes. I use soaker hoses to water at the ground level and made sure to give them proper spacing but still the mildew is an issue. I am constantly cutting off coated and yellowing leaves. I have tried using organic copper sprays to no avail. My Dad suggested a diluted bleach solution sprayed on the plants. I was afraid it would burn the leaves but it seems to be helping. Hopefully it does the trick otherwise we are going to lose the plants and my dreams of zucchini bread, soup and freezer bags full just won't be.