Zucchini leaves turn white when the plant is infected with a fungus called powdery mildew. Powdery mildew is common on squash and other vegetables, especially in late summer.
Powdery mildew requires a live host to spread. The fungus first appears as small white patches on the leaves, and then spreads to the undersides of the leaves to the stem and sometimes to the flowers. It grows by reproducing spores, which are the white powdery substance that is visible on the plant. Powdery mildew often strikes the plant towards the end of the growing season and a mild case does not kill it. It is possible to pick the white leaves off the plant; the zucchini is still edible. A more serious infestation may reduce the yield of the harvest and cause the fruit to lose flavor.
Powdery mildew is controlled by using a homemade spray of baking soda and water or by using an insecticide. If the case is severe, it is best to consult a professional or a gardening center. The best solution to powdery mildew is to prevent against it. It is important to keep plants healthy, well watered and not overcrowded. Many varieties of plants are resistant to the disease.