Powdery Mildew

My experience with zucchini and cucumber is they tend to be prone to powdery mildew.  Powdery mildew in the cucurbit family is is a fungal disease caused by Sphaerotheca fuliginea.  In severe cases it will destroy the entire plant as well as spreading to healthy plants.  As with any plant disease recognizing and treating the problem early can make a huge difference as to whether the crop is saved