White spots on zucchini leaves may be part of the natural leaf coloration, or the spots may be powdery mildew fungus.
Some varieties of zucchini develop white splotched markings on the leaves as they get older. If the plant is growing and producing zucchini and has leaves of normal size and shape, the white spots are probably the natural color. You can examine the spots; if you cannot rub them off the leaves, the coloration is natural.
If the white spots are powdery mildew, they look similar to a dusting of flour on the leaf, and you can rub off spots. The cause of the spots is spores produced by the fungus.
Powdery mildew grows in moist, humid conditions. Water your garden early in the morning so that the air around the leaves is dry by nightfall. Make sure the plants are far enough apart for plenty of air circulation. If powdery mildew develops, remove the infected plant parts to prevent further spread of the disease.