What’s Causing the White Trails on My Zucchini Leaves?

I don't think it's powdery mildew, though. That stuff usually looks like someone sprinkled powdered sugar on the leaves. What you've got seems more like a natural pattern. To check, try wiping it off gently—if it comes off easily, it's probably mildew. If it stays put, then it's likely just how those particular zucchini look. Some varieties can have those color variations.

I can't say for sure without a picture, but those white trails might be a type of powdery mildew, which is pretty common on zucchinis. A good solution is to spray some Neem oil to keep it from spreading. Also, remember to water your plants in the morning; that way, the leaves dry off quickly and you avoid creating a humid environment that encourages mildew. Water at the root level too, to keep the leaves as dry as possible!