Got Powdery Mildew on Zucchini, Squash and Pumpkin Plants?
This happens to all our pumpkins, squash, zucchini and cucumber plants every summer late in the season, treatments like milk or baking soda sprays don't seem to help. We usually prune off the worst affected leaves to give it more air circulation, but have learned to accept that this is the natural late-season stage here in the West. So don't worry, the squash, zucchini and pumpkins will still ripen!
One thing about powdery mildew: squash and zucchini may not produce as much as earlier in the season, so here's our suggestion:
This year, since we knew Powdery Mildew was coming, we decided to plant some fresh Black Beauty Zucchini seeds in late July, and boy are we glad we did, because we have three new robust & healthy zucchini plants that are already starting to produce, and they are as happy as can be – they are fairly close to the mildew affected plants, but don't have any on their leaves – so perhaps it's a life cycle thing in our climate. Bottom line, don't worry! This late season squash & pumpkin affliction is not your fault, just nature's cycle.
Below: Check out this healthy late-July planted Black Beauty Zucchini planted from seed just one month later it's already fruiting! Wow! 
We also TRY to water under the leaves with all these plants so the leaves don't get wet, but in late summer that difficult as they plants spread and grow huge... but despite this, the plants always get powdery mildew in both our home garden and our community garden plots down the street, so we're just learning to accept it! Note: We also get water on the new zucchini plants that we just planted in July, and they are green and happy with no signs of mildew, so I really think it's the end of their life cycle here kind of thing.
Grow the amazingly fast growing Black Beauty Zucchini and you'll be harvesting zukes in as little as 55 days! So doing a succession planting in mid-summer (such as July) will likely produce a great fall crop in September if the cold weather doesn't come too early. :) Happy growing!
