Powdery mildew on zucchini leaves?

(Question)

There is some white spots on my zucchini leaves. I was wondering if they were infected by powdery mildew. How can it be treated?
Thank you so much for your help!

(Answer)

Thank you for asking about your zucchini leaves’ powdery mildew . From your photo it does seem that powdery mildew is your culprit, but the good news is that your flowers look healthy and if you can control this now, the fruit should not be affected .

Squash leaves are notoriously sensitive to powdery mildew, which is an air borne fungus appearing at first as opaque , grayish white specks that grow and expand to cover the whole leaf. This is one of the most common issues with zucchini and arises in warm daytime weather when nights are cool and humid such as those we have experienced recently . The fungus thrives in poor air circulation so be sure to leave as much space between plants as possible . Perhaps thinning your plants will help. Can you trellis them to keep them off the ground and more in the sun ? As much sun as possible is good.

According to the book, What’s Wrong With My Plant (And How Do I Fix It?) by plant pathologist David Deardorff, PhD, and naturalist Kathryn Wadsworth, the best approach is prevention by removing infected plant material (and putting it in the garbage, not the compost), mulching to prevent water from splashing around and spreading the fungal spores, providing air movement around the individual plant, and choosing resistant cultivars.

When you water your garden make sure to water only the roots of the plant, keeping the foliage dry and water early in the day so that the water has a chance to evaporate. Remove plant residue in the fall to reduce the inoculum source .

You can treat a powdery mildew by spraying healthy leaves with a fungicide. It should be applied early in the season. The plant will likely need repeated applications every week or ten days but be guided by close observation of your plant.

If the majority of the leaves have powdery mildew, it might be too late to treat and the plant should be removed but if only some leaves are affected, and there are enough left to provide photosynthesis and to keep the fruit from sunscald , it is worth trying some of the above tips. From the looks of the photo , you should be in time to enjoy a harvest of delicious zucchini . Thanks for contacting us and bon appetit!

For more information on treating powdery mildew please see this website:
https://extension.umn.edu/plant-diseases/powdery-mildew-flower-garden