Zinnias are very prone to attack by mildew fungus. This gives the appearance of powder on the leaves as you describe. Some seed strains are more susceptible than others. The Blue Point and Profusion series are examples of more resistant strains. Also try Zinnia angustifolia Star series, which are low growing but excellent at th front of the border. To reduce mildew spreading all over your zinnias, spray with a solution of 1/2 teaspoon baking soda in 2 quarts of water with a few drops of liquid soap. Mildew causes defoliation and weakens the plants as well as being unsightly. Plants that are stressed with uneven watering and lack of fertilizer are usually the first to succumb.