Answer: Powdery mildew is a fungal disease and spreads by spores, either wind blown or splashed by water. If one part of a plant develops powdery mildew, it can spread to other plant parts. Some of the newer zinnia varieties have been developed with mildew resistance in mind. The "Pinwheel" series does very well in this regard, and the "Profusions" have done especially well for me in terms of mildew resistance.
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