There's white powdery stuff all over my squash and cucumber leaves
Powdery mildew is the one leaf disease that doesn't need wet leaves — it likes humid air and crowded plants. By the time you see it late in the season, it has usually already done the job you needed the plant to do.
What actually works
Crowding is the condition. Thin, trellis, and don't plant the next squash in the same still corner.
Widely available and free to choose. In a bad-mildew climate this is the whole answer.
On a plant that has been producing since July, mildew in late season is the plant finishing, not a crisis. Spraying it then is effort spent on nothing.
This is the one folk remedy with real trial evidence, specifically against powdery mildew, and it's modest. Worth trying on a few prized plants; not a system.
Also searched as
powdery mildew · white film on leaves · mildew squash · dusty leaves · leaves turning white · fungus on cucumbers