Diagnosis
Diagnose: Powdery Mildew on Zucchini
Use these observations to decide what to check next.
Powdery mildew when raised white patches cross the leaf veins
Rub one small white patch with a damp dark cloth, then look at the leaf pattern around it.
Powdery mildew leaves a loose white or gray residue and forms irregular colonies that cross veins, while healthy Zucchini silvering is flat inside the leaf and repeats in sharp angles along the veins. Turn the leaf over as well because true colonies can occur on both surfaces and often begin on older shaded leaves.