Your problem is likely powdery mildew, an opportunistic but difficult disease. Powdery mildew first appears as pale yellow spots on stems, petioles, and leaves. These spots enlarge as the white, fluffy mycelium (fungus) grows over plant surfaces and produces spores, which give the lesions a powdery appearance. If not controlled plants may die. There are remedies ranging from selection of resistant varieties to sprays of neem oil in the early stages. At this stage, suggest you take off the affected leaves, begin spraying the plants, and try to arrest the disease to save the crop. When you do spray, spray in the late afternoon or early evening to prevent sun scald on the foliage. It may be too late, but we hope you can save your zucchini plants.